This guide is here to help you understand what manual transmission fluid is, what it does, and how you can select one objectively. I wrote this guide after much lurking on the Bob Is The Oil Guy forums and talking with various racers, chemists, and reading countless pages of research and personal experiences on various automotive websites.
You should read this guide if you are interested in selecting a different fluid or are simply curious about what’s been in your transmission this whole time. Nothing in this guide should be over anyone’s head. Believe me, the chemistry and science inside automotive fluids is overwhelming. I’ve done my best to pick the bits and pieces that I feel are most relevant to us and summarize them here.
If anyone is willing to host these photos and the Excel file somewhere, please PM me.
Terminology
MTF: “manual transmission fluid”, a common abbreviation.
GL-x: “gear lubricant”, a class of lubricants. The number at the end specifies which exact specification the lubricant meets, such as GL-4, GL-5, etc.
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.
TA/TX: “transaxle”, often used interchangeably with the word transmission.
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.
The B6 & B7 manual transmission
Without getting too technical, the B6/B7 S4 manual transmission is a Getrag M6S unit coupled to a Torsen T-2 quattro permanent 4WD system. This unit serves dual-duty as a center differential to regulate power between the front and rear axles.
Due to its size, the transmission requires 3.5 quarts of GL-4 75w90 fluid for a change. The key piece of information here is GL-4. You cannot, I repeat CANNOT use a GL-5 fluid.
Why is GL-5 bad news for us? It has to do with the additive packages. GL-5 is not simply the “new version” of GL-4. It is a different specification all together. GL-5 is not necessarily better, it’s simply a spec for different usage.
GL-4 fluids contain about 40-60% as much additive as a GL-5 would. These additive packages can be corrosive to certain metals. While the transmission would function with GL-5 inside, it would wear itself out very quickly. Some cars can run GL-4 or GL-5, ours cannot.
Let’s throw a curveball in here though – can a fluid be GL-4 AND GL-5 compliant? The answer is yes. Many fluids are now made to be “anti-corrosive” or “non-corrosive” meaning that they will satisfy both specs. Don’t automatically assume that’s the case; you need to look for the bottle to explicitly state that it won’t corrode things. Often times it will say “brass synchro-safe”.
Sometimes the bottles are marked as GL-4+, meaning it satisfies GL-4 specs and up.
Types of fluids
There are 3 primary classes of MTF fluid: conventional, semi-synthetic, and full-synthetic. Our cars call for synthetic only. VW/Audi has specified synthetic-only since the year 2000. Much like engine oil, synthetic oils last considerably longer and do not break down as quickly under stressful conditions. Would a mineral oil work? Probably, but no one wants to be changing the fluid every month.
So we are looking for 3.5qt of synthetic GL-4 at 75w90. Easy enough, right? If you’re still reading at this point…you know there’s more coming.
As I briefly wrote above, the term synthetic is very vague and poorly defined in the United States. Throughout the rest of the world, synthetic means that it is 100% man-made. There was a dispute over this a while back and that’s why a non-synthetic or partial-synthetic can be sold as synthetic over here. Frustrating, I know. A true synthetic will be marked as being either a PAO or ester.
Does that mean you must run PAO or ester-based MTF? No. There are plenty of synthetic MTFs that will work just fine and will cost less. PAO and ester-based MTFs are more expensive but protect better. In my opinion, when it comes to a transmission costing thousands of dollars, I’ll spend the extra money to protect it. I may never see the benefits of the better fluid but I sure as hell don’t want a $2000 repair bill because I wanted to save $20 on the fluid.
Specifications and what they mean to you
There are three key specs that I’ve selected to catalogue since they’re most indicative of the fluid’s performance. I wouldn’t advocate selecting an MTF 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 MTF 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 MTFs that have the same VI. VI can be derived from the next two values, actually.
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 low number means the oil flows more freely at the given temperature and hence less power is lost due to friction. But lower numbers mean the oil has less resistance to high temperature. Higher values mean the oil is thicker and hence dampens performance. Thicker does mean better high-temp protection, usually. Selecting something too thin might improve transmitted power and gas mileage but could cause less protection during extreme conditions. Conversely, an oil that's too thick will cause a lot of drag which generates more heat in the gearbox.
Viscosity @ 100 C is the same as above, only measured at 100 C (212 Fahrenheit).
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 transmission 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 with a good viscosity index (an indicator of high-quality basestocks) and then select something that is thin at lower temps but not too thin when hot.
Caveats
Below is a compiled list of GL-4 MTF fluid. Any of these should work in our transmissions. There are some blanks since some data is not released by the manufacturers. There are some fluids not listed because I have no data available. If you find more, please send it my way and I’ll update the list.
Caveat 1: these do not measure subjective feel in any way. Some fluids may “feel” better than others.
Caveat 2: if your transmission is in poor shape, no fluid will magically fix it. Some fluids may help disguise the problem, but busted synchros do not suddenly re-grow new metal.
Caveat 3: some of these fluids are not easy to come by. Some are not even sold in the US and will need to be imported (probably at great cost). I’ve tried to mark this wherever possible.
And finally…when in doubt, go for the OEM fluid. OEM MTF is a decent product. The OEM stuff is actually a European-only formula sold by Castrol and rebranded for Audi/VW. You can’t even buy it here in the states except at the dealer. Audi didn’t pick a crap fluid as factory fill. Sadly, it costs a fortune.
ALL SPECIFICATIONS:
Conclusions & closing thoughts
Any of these oils meets the GL-4 spec and are technically safe to run in the transmission. But the level of protection they provide is very wide, as is their price and availability. I hope this guide has helped you be a better consumer and become more educated on this topic.
Here’s your Cliff’s Notes:
• Motul Gear 300 and Lubro-Moly Hochleistungs-Getriebeol are the two best buys in terms of objective performance and performance-per-dollar. Both are easy to come by.
• The most well-rounded fluids are Lubro-Moly Hochleistungs-Getriebeol, Amsoil MT & TA Gear Lube, and Red Line MT-90.
• The best high-temp performer is the Maxima Syn by a hair over the Lubro-Moly.
• The best low-temp performer is Motul Gear 300, but it's just barely better than OEM.
• OEM fluid is actually very good, it's just way overpriced.
DISCLAIMER: No guarantee or warranty is given to the accuracy of this information. I have done my best to verify its accuracy but I am not accountable for anything bad that happens as a result of this information.
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