“What I Learned Today” With Jeff Smith: Thin Isn’t In

Thin Isn’t In

We tried a new version GL5 gear lube in our old school Super T-10 thinking that the more modern lube might help with reducing gear oil temperature and maybe even improve the shift quality of this transmission. Our idea was that this new gear lube is more slippery at operating temperature. Our idea didn’t work so don’t try it. The older, brass synchronizers are designed to use a thicker, more viscous gear lube that is specified as a GL4 lubricant.

GL4 lubricants generally don’t use additives that are designed for use with limited-slip differentials. Most often, rearends specify a GL5 lubricant that is designed to be more compatible with the sliding friction present in hypoid style gears, which is very different from the lubes required by older manual transmission using the brass style synchronizers.

Among the companies that offer a GL-4 gear lube compatible with older transmissions using “yellow metal” or brass style synchronizers are Amsoil, Brad Penn’s PennGrade, Red Line, Royal Purple, Richmond Gear, and StaLube. We’ve limited our attention to these lubes but there are certainly others. These GL4 lubes are the best choice to use with older manual transmissions.