EP in GL4 vs GL5

[rotorwrench]

When they start popping out of 2nd gear, they may be letting a person know that the bushings are going. How fast they go depends on a lot of factors. If you eliminate the acidic nature of the high EP in GL5 then that will be one less factor to worry about. If a person doesn't care then by all means, use the GL5. Use of GL5 in an R-10 overdrive transmission can cause sprag clutch slippage so that's another reason to limit the EP additives if so equipped.

SAE 50 Motor Oil is the same viscosity as SAE 90 gear lube so it can be used in a pinch. I use Aeroshell 100 mineral in my overdrives.