Up until 2017, we had been recommending MT-90 for Vanagon manuals because it is the only high quality, synthetic GL-4 oil available. GL-4 is what VW called for back in the ‘80s, when it was commonly available and the standard for many applications. Nowadays, the standard is GL-5, which is a superior lubricant. Gears and bearings just last longer with GL-5, plain and simple. So, why not switch?
Well, there has been some concern regarding the chemical composition of GL-5 that could result in harm to the brass synchro rings in the Vanagon transaxle, and/or that it was too slippery for the Vanagon synchro ring design and would cause shifting issues (grinding). These arguments never made much sense to me, as I have never seen a synchro ring that was not yellow brass and of the same conical design just like in every transaxle I have ever torn apart. In fact, even modern manual transmissions use the same design synchro rings, and all currently run on GL-5.
So, we decided to switch to a top quality GL-5 in several of our in-house Vanagons starting in 2015 - including our LeMons/NORRA race van - to try it out. We have found that they all shift no differently than your typical Vanagon manual with GL-4. Also, we have seen no evidence that the GL-5 has any effect on the material the synchro rings are made of, which is not surprising.
These test results, and discussions with transaxle parts suppliers, led us to switch from recommending GL-4 to GL-5 for all Bus, Vanagon and Eurovan manual transaxles, as well as the final drive sections of all equipped with automatic transaxle.
So, which brand is the absolute BEST, HANDS DOWN, GOTTA HAVE IT, NOTHING ELSE WILL WORK?! Clearly, you can put ANY brand oil in your transaxle, and as long as it is the correct viscosity and type: it will work. The question really is will it make a difference as far as how long it will work (how long the trans will last). Lots of shops that build transaxles might tell you they know what THE BEST oil is, and it is not uncommon for many to require a particular brand to maintain the warranty—GoWesty included. Any shop that tells the oil they use is for sure the best, superior to any other—is just talking out of their ear! It most definitely is the best they have used, but no way they can possibly have used them all and done a proper, all things equal comparison. That is just not feasible for many reasons.
So, how do we know what we sell is THE BEST? We don’t. All we know is that we have build thousands of transaxles and have used the same brand since the beginning, and we just do not have lubrication-related failures under normal use. So, while we cannot say this stuff is the best, we can say it is damn good and works great. And there is zero doubt there are other brands that are just as good—we don’t have experience with any others. All we can say is that what we have been using works great, so we are going to stick with it, lest we jump out of the transaxle and right under the wheels!
Your best bet is to stick with top-quality, full-synthetic, brand-name oil and change it regularly.