This guide is here to help you understand motor oil. Motor oil plays a key role in lubrication, cooling, fuel economy, and emissions yet is one of the most misunderstood things in a car. I wrote this guide after doing a lot of research and talking to folks on the Bob Is The Oil Guy forums. My experiences talking to various chemists, racers, and other enthusiasts are also included.
Read this guide if you’re about to change your motor oil or are simply interested about what’s been coursing through your car’s veins all this time. This guide is somewhat techy but nothing that the average car enthusiast can’t follow along with. I’ve tried to pick the best bits of information for this article.
Be an educated consumer! Know what you are spending your hard-earned money on. Understand the concepts behind motor oil so you can make educated decisions instead of blindly buying into hype.
PAO: “polyalphaolefin”, a man-made (synthetic) version of mineral oils. This is sometimes known as a group 4 oil, not to be confused with GL-4.
Ester: for all intensive purposes, they’re fats. These are a synthetic item used in extremely high-end lubricants. Variations include diesters and polyesters. This is sometimes known as a group 5 oil, not to be confused with GL-5. Esters happen to be very stable at high pressure and high temperature, which is important for motor oil.
Synthetic: in the US, this doesn’t mean much. Throughout the rest of the world, this equates to “man-made”.
Semi-synthetic: a mineral/conventional/dino oil that contains no more than 30% synthetic oil mixed in. The thinking here was you can get the best-of-both-worlds.
Mineral/conventional/dino: naturally-occurring oil that is refined and processed into various lubricants, such as engine oil, gear lube, MTF, etc. The “dino” portion comes from the fact that the oils are the byproduct of decomposing dinosaur remains.
Motor Oil: A Hard Worker
Motor oil serves many purposes in the motor. Its primary purpose is of course lubrication (and protection) of all the moving parts and surfaces. We don’t need to go into too much detail on this right now because it should be fairly obvious that we want to avoid metal-to-metal contact in the motor. Let’s take a look at the other things that you may not know motor oil does.
Another role motor oil plays is that of cooling. While it’s true that the cooling system is the primary system in charge of keeping temperatures under control, the motor oil is closely related to this as well. As oil is pumped through the motor, it comes in contact with the hot metal surfaces. Through conduction it absorbs some of that heat and carries it away from the valvetrain, pistons, etc. The enormous oil capacity of these motors is what provides additional cooling above and beyond what the car’s fans, radiators, and coolant already provide. More oil means more mass to absorb the large amounts of heat this engine generates.
Motor oil is also important to fuel economy. This is an extension of how it lubricates. A motor that is better lubricated will of course require less effort to run and therefore provide better gas mileage. It is true that different oil viscosities and weights will actually impact fuel economy. Changing oil types will not make our lousy 14mpg turn into 24, but it may bump city economy up by 1-2mpg. There are also special additives in oil that improve fuel economy by further reducing friction. More on this later.
Lastly, motor oil plays a part in emissions. There are additives in gasoline and motor oil designed to reduce certain byproducts of combustion. Some oils are better at this than others. To some this will not matter, but if you are making efforts to be more “green”, you can make smarter choices about the oil you purchases.
First, let’s take a look at one of the most misunderstood specifications oil carries: the viscosity rating. This is a numerical system was developed by the SAE to designate how viscous it is. Viscosity is a “property of a fluid that resists the force tending to cause the fluid to flow”. The test is a measure of how fast a certain quantity of oil takes to flow through a certain size opening. To apply this to what we’re talking about here, think of it in terms of how quickly it can squeeze between the metal parts of your engine.
There are two basic grades of oil: single-grade and multi-grade. Single grade has no viscosity modifiers. You’ll see this marked as simply “30 weight” or “straight 30” oil. Our cars cannot use this so we won’t spend too much time talking about it.
Multi-grade is what we’re interested in. Multi-grade means that the oil performs across a wide-range of temperatures. The viscosity will be very high when cold and very low at operating temperature. It’s a lot like running two separate oils. This is accomplished via proprietary viscosity modifier additives. Multigrade oils are listed with numbers like 0w40, 5w30, 10w30, 20w50, and so on. Let’s break down what these two numbers mean.
We will use 5w40 for our example. The first component, “5w” describes the oil’s viscosity at cold temperatures. The W is short for “winter” and NOT “weight”. This is a common mistake. The second component, “40”, describes the oil’s viscosity at operating temperature. So 5w40 means that at cold temperature it behaves like a 5-weight, and at operating temperature it performs like a 40-weight. Remember the test we talked about up top? A 5w40 would pass both the 5 AND 40 tests!
So why does this matter? Well, for vehicles driven year round and across a wide range of temperatures, we want an oil that pumps quickly and protects well when it’s cold, and still lubricates well when it’s very hot. A 0w40 will do much better in cold temperatures than a 5w40, for example. A 20w50 will perform poorly at low temperatures versus a 5w40, but will do very well at high temperatures. A significant amount of engine wear occurs when the motor is very cold so we want maximum protection there.
Audi specified a 5w40 as factory-fill since it’s a good balance between cold and warm temperature performance. A 5w40 will pump well when temperatures are below zero but won’t suffer when you’re at redline in the middle of a summer day. 5w30 is also acceptable and could yield slightly better gas mileage.
Something to note is that the given viscosities are going to be a range, as evidenced by the chart below. Not all 5w40s are the same viscosity. Some will be “heavy” and some will be “light”. In fact, one of the OEM oils, Castrol SLX Professional OE, is a “heavy” 5w30 that’s almost a 5w40.
Basestocks are the starting point for all motor oil. The basestock is refined and mixed with other chemicals to turn out the final product. Not all motor oil uses the same basestock. Fortunately, there is a classification system that categorizes the various bases and defines where they come from and what they contain. This was created by the API (American Petroleum Institute).
Group 1 and 2 oils are the low-grade conventional oils. These come from lower quality basestocks and are not synthetic. This is the cheap oil you see for $1-2/qt at the local auto parts stores. We sometimes call these dino or conventional or mineral oils.
Group 3 oils are where things get hazy. Group 3 is still conventional oil – but it’s marketed as synthetic in the United States. How is this possible? Well, let’s just keep it simple and say that after a lawsuit many years ago, it was decided that synthetic does not refer to the source of the oil. Group 3 oil is still very good and is essentially the highest grade of conventional oil available. It is classified as a synthetic because it undergoes a process known as hydrocracking or hydroisomerizing. Again we’ll keep it simple and just say that the oil undergoes special processes that further improve its properties and performance. Sometimes grade 4 or 5 oils are mixed in to boost it even further. Group 3 oils will run you about $4-6 a quart.
Group 4 oils are true synthetics. They’re often called “PAOs”, or polyalphaolefins. Now we are talking about the truly excellent oils. Expect to pay $5-9 a quart.
Group 5 oils are the top-of-the-line lubricants. These often use things like synthetic esters (fats) for even better performance than a PAO oil. Prices here are even higher, usually above $9 a quart.
Again let’s ask ourselves why this matters. It matters because synthetic oils are not all created equal. As it is, the term synthetic can be a bit of a misnomer in the United States. Generally speaking, better basestocks mean better oil…and an empty wallet.
The VW 502.00 specification
Like most other manufacturers, VW/Audi developed some specifications for how motor oils used in their cars should perform. If you’re under warranty, I suggest you stick with oils that explicitly meet these specifications and are approved. There are plenty of choices out there – you do not need to run the dealer-supplied oil. There are even oils out there made just to this spec. Motul, for example, sells something called “Specific 502” that was made just for VW/Audi. They do the same for BMW as well.
The 502.00 specification lists a 10,000 mile change interval and a 5w30 or 5w40 rating. This means that in general, a 502.00 approved oil is good out to 10,000 miles. Continue reading as we dive into change intervals a bit more.
You can find current lists of approved oils by doing some quick Google searches. The list changes periodically as old oils are dropped and new oils are released.
There is a difference between “meets/exceeds VW 502.00” and “VW 502.00 approved”. The oil may technically meet VW’s requirements, but VW may not approve it. The approval means VW has specifically said “yes, we certify this oil for our cars”. There’s tons of oils that may not be certified but could outperform the certification too!
Change Intervals, aka “Why You Want Synthetic Oil”
Let me guess – your dad, Jiffy Lube, the oil companies, and everyone you ever knew told you to change your motor oil every 3,000 miles or 3 months, right? Sadly, this is somewhat of a myth. 40 years ago when motor oils were pretty much all conventional group 1/2 10w40, the oil didn’t hold up well after 3,000 miles. Hence the 3,000 mile myth was born.
Then why is it still around? Basically because Jiffy Lube and the oil companies love taking your money. They’ll continue to explain that oil doesn’t work past 3,000 miles, it gets too dirty, engines are getting very complicated, and so on. This is a lot of exaggeration and conjecture. Note how many cars actually have an interval of 4,500, 5,000, 10,000 or more these days.
The reason we change oil is two-fold. The first is that the oil breaks down. Repeated heat cycles, burn-off, and contamination actually change the chemistry of the oil. Conventional oil does so very quickly. Synthetics do this at a much slower rate. The other reason we change oil is because it gets dirty. Assorted metals, minerals, gas, coolant, and moisture get in the oil. The oil filter will catch most of this but it is not 100% effective. The rate at which oil gets dirty is largely determined by the engine and not the oil itself.
Let’s say we have two identical cars. One runs full synthetic oil and the other runs conventional oil, and both cars have the exact same filters. By 3,000 miles, the synthetic and conventional oil will be about the same in terms of dirtiness. If we were to test the viscosity at this point, there would be a significant difference. The synthetic oil would still test as the exact same viscosity as when it was new (or extremely close). The conventional oil will have started to break down and actually change viscosity. It may have lost its multi-grade rating or could have sheared to behave like completely different oil (for example, a 10w30 could become a straight 30 or worse).
So by that 3,000 mile mark, the conventional oil should probably be changed. The synthetic, although dirty, is still performing like it should. This is why the change intervals for synthetic are often longer.
Can you use conventional oil? The caveat is that the oil will not stand up to abuse for as long and will need to be replaced much more frequently. For a motor of this complexity and performance, I would stick with synthetic per VW’s recommendation.
How do we know when to change the oil? Is the manufacturer specification of 10,000 miles or 12 months adequate? Is that too cautious, or not cautious enough? The best way to tell is to get the oil analyzed.
Companies like Blackstone Labs and Dyson Analysis will run a small quantity of your used motor oil through machines that spit out exact reports of what the oil looks like. You will receive a breakdown of all the contaminants in parts per million, the oil’s viscosity, how much additive is left, etc. From this you can determine if the oil was “used up” or if it could’ve been left in longer. Unfortunately the first change you make might have been too soon. But spending $25 on the analysis could help you save a lot more down the road.
So is Audi’s 10,000 mile/12 month recommendation a good one? The easy answer is yes, for the typical driver. The car’s large oil capacity means there is a lot of oil to absorb those contaminants and it will take longer to start breaking down. Quality synthetic oil should be able to withstand the 10,000 miles of use.
For atypical use, such as racing and repeated short trips, the 10,000 mile mark might not be often enough. To be safe, some owners change twice as often (every 5,000 miles or 6 months). Again, the only way to know is to get a sample analyzed.
An interesting bit of info the analysis will give is something known as the TBN, or total base number. This value is a measure of how much reserve alkalinity the oil had. Through the process of combustion, acids are formed. The TBN of a particular oil tells you how good that oil is at neutralizing the acids. As the oil is used, the TBN slowly drops. It may start with a value around 9 and fall to 3.8 by the time you change it. As the number approaches the range of 1-2, it means the oil is going to be unable to control acid formation and it should be changed. Quality oils will generally have a TBN of 7 or better, with some being as high as 12. If your report shows a TBN of <2, you probably needed to change the oil sooner. If it was above 2, you could probably have left the oil in longer.
Oil analysis is often misused to measure the motor’s overall health. While it is true that you can get some insight as to what’s happening in the motor, it should not be the sole tool used to diagnose a problem. Your report might show high aluminum content – scary, since that could mean the pistons are wearing excessively. But it could also just be some leftover aluminum from the transport container.
Relating this back to the 502.00 specification, it means that the oil is sufficiently resilient to withstand 10,000 miles of punishment. A 502.00 oil also has a higher TBN so that it can continue to neutralize acids until 10,000 miles.
Specifications and what they mean to you
There are several specs that I’ve selected to catalogue since they’re most indicative of the fluid’s performance. I wouldn’t advocate selecting an oil based on a single number, but rather, a weighted-average method. I’ll explain that later.
Viscosity index is how well the oil resists a change in viscosity across the temperature range. A higher number is better here. As the oil heats up, we want it to stay as close to its original viscosity as possible. The lower that number, the more wear that occurs. This is a unit-less value. One caveat to this number is that it only shows you the viscosity/temperature relationship between 40 and 100 C. Performance at 0 C may be very different in two oils that have the same VI. VI can be derived from the next two values, actually. The VI can be improved by adding those “VI improvers” (VIIs) but that comes with a caveat: the VIIs often break down at very high temperatures and this causes the oil to shear. Shearing means the oil’s viscosity begins changing. In general, better oils need fewer VI improvers.
Viscosity @ 40 C is the kinematic viscosity of the fluid at 40 degrees Celsius (104 Fahrenheit). The kinematic viscosity is essentially “the amount of time, in centistokes, that it takes for a specified volume of the lubricant to flow through a fixed diameter orifice at a given temperature”. A lower number is better here. A low number means the oil flows more freely at the given temperature.
Viscosity @ 100 C is the same as above, only measured at 100 C (212 Fahrenheit). The chart above actually shows you the acceptable range for a given oil viscosity. A 40-weight oil, for example, must have a KV @ 100C between 12.5 and 16.3. For a 30-weight, it is actually 9.3-12.5. So not all 5w40s are created equal.
Selecting an oil purely because of a single value is going to lead to problems. If you select the oil purely based on its viscosity at 100 C, ignoring all other values, you could be in for a world of hurt. It might perform horribly at lower temps, meaning that the motor will wear excessively until its warmed up. Conversely, selecting it based only on the 40 C value means that it may protect well when cold, but will cause excessive wear at very high temperatures.
You need to try and balance these numbers with your usage patterns. Does the car see lots of track time? You may want to give more weight to the 100 C figure since that’s an indicator of high-temp performance. Does the car just see light duty around town? Perhaps the 40 C number is more useful since you’re not thrashing the transmission and running it near its limits for 20+ minutes.
If you’re like me, and you want the best all-around fluid, get the lubricant that has the highest viscosity index and the lowest kinematic viscosity values. Open your wallet wide for this.
There are a few other figures worth discussing such as HTHS, TBN, and NOACK. We already talked about TBN up top in the change interval section.
HTHS is “high temp high shear”. This is another SAE test that measures oil viscosity under high temperature high shear conditions. In particular, this relates to valvetrain and bearing regions. Higher is better; much like the VI rating, this is a strong measure of how well the oil resists changing viscosity. Again, the SAE sets a minimum value. For a 40-weight, it must have a minimum value of 2.9 (3.7 is the minimum for 15w40 and some other less common viscosities). For 30-weight, the value is also set at a minimum of 2.9. Whereas the VI can be artificially boosted with additives, the HTHS number is much more difficult to inflate. Sadly, many oil manufacturers do not publish this data. If you are interested in it, you may need to contact them directly.
The last figure is also very elusive: NOACK volatility. This is an ASTM test that determines evaporation loss of oils under high temperatures. As temperatures rise, oil will begin to boil off some of its lighter weight molecules. This leaves behind the heavier stuff. Once this begins happening, oil consumption goes up and the viscosity goes up as well. In turn, the oil does not circulate as well. Lower values are better, as it again means the oil resists changing viscosity under extreme temps (150 C for this test). Lower values mean the oil retains its performance well under stress.
To wrap this section up, keep in mind that these numbers don’t paint the whole picture about a given oil. What separates one oil from another is the additive pack it has. Some additive packs are loaded with VIIs that just artificially improve the viscosity index. Others may have components that are particularly good at resisting burnoff. The additive packs are a trade secret, though you can often speculate on their formulations based on various test data. Don’t just buy an oil because it has an amazing KV value or because it has an insanely low pour point.