Transmissions and the fluids they use have come a long way over the last 50 years. One of the most misunderstood uses of fluid today is ATF in a manual gearbox. There are a lot of old school hot rodders that can’t wrap their head around why you would use an automatic transmission fluid in a manual transmission. As manual transmissions evolved over the years to include lined synchronizer rings, overdrive gears, and tighter bearing tolerances the type of fluid used had to evolve with it. ATF fluids flow better in cold temperatures, create less drag on moving components for more efficient power transfer, and do not deteriorate the friction materials that were added to the synchronizers.
Today you will rarely see a manual gearbox that doesn’t use an ATF or what now has evolved into a specific variation of ATF referred to as Synchromesh. Hypoid gear oils are still used in the older manual transmissions that called for them, but using them in the new transmission is a no go as the hypoid gear oils are designed to withstand sliding actions at the meshing interface of bevel gears. Some hypoid oils contain additives that are corrosive to certain non-ferrous or “yellow” metals that are used in transmission parts like bushings and synchronizer rings. Also, the friction coefficient of some hypoid gear oils may be incompatible and can cause poor shifting. Both of these conditions can result in premature or excessive wear and failure in the newer manual transmissions. We exclusively use the Royal Purple Synchromax in all our Tremec transmissions. After years of trial and error with other brands and styles of fluid we’ve found that the Royal Purple blend offers the best overall performance and shift feel out of the other fluids on the market.
The choices between the standard mineral oil fluids to the array of new synthetics can be a little overwhelming. There are good arguments to go in both directions, but the reality is that most all newer components are going to be designed to work with a fully synthetic fluid or synthetic blend of fluid to reduce frictional power losses. Also keep in mind that all synthetic fluids are not created the same as some are mineral based with synthetic additives and others are a fully synthesized version. Take the time to research which fluid is recommended or will work best for your particular situation as most manufacturers will void the warranty if the incorrect fluid is used.