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.
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 rear differential
The rear differential is the final piece of the drivetrain. It contains the 3.889:1 final drive gear.
The differential requires approximately 2.0 quarts of GL-4 or GL-5 75w90 fluid for a change.
Unlike the transmission, we can safely run any synthetic GL-5 gear oil. That opens the door to countless product choices. Although GL-4 is acceptable, your differential will perform best with GL-5. The GL-4 additives simply do not hold up to extreme pressures as well. Remember that the rear differential is effectively multiplying the motor’s torque another four times. At redline in 1st gear, the differential is putting over 4,000lb-ft of thrust to the pavement. You want the maximum protection and engagement here.
Types of fluids
There are 3 primary classes of 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 2.0qt of synthetic GL-4/GL-5 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 a gear oil based on a single number.
Viscosity index is how well the oil resists a change in viscosity across the temperature range. A higher number is better here. As the fluid 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 fluids 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 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).
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.
Caveats
Below is a compiled list of GL-5 gear oil. Any of these should work in our differentials. 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 differential is in poor shape, no fluid will magically fix it. Some fluids may help disguise the problem, but busted teeth do not regrow 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 differential fluid 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-5 spec and are technically safe to run in the differential. 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.
My recommendations
• If you’re under warranty and feel the itch to change it, stick with OEM.
• A daily driver can stick with most of the store-bought fluids like Valvoline, Castrol, etc. They’re cheap, readily available, and they’re pretty close to the OEM fluid weight for much less money.
• A daily driver that is really thrashed could move up to something like Motul Gear 300, Eneos MT, or Maxima Syn Gear. They’ll resist thinning out at really high temps and won’t give up too much up the way of cold-temp protection.
• A car that sees extensive track time should probably consider a thicker 75w90 and/or a fluid with a very high viscosity index so as to maintain film strength under high temperatures.
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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