on my 41 ford with the marmon herrington 4x4 i filled my drive line with modern GL-5 fluid. went out for a ride and the drive line was real noisy and the transfer case got real hot. checked the manual and it specified sae 140w GL-3. GL-3 is for mild extreme pressure. i was able to find it (Summit racing). the GL-3 lube made a huge difference in driveabilty. note the modern oil i originally used said it covered GL-3 but was EP. after looking in to it the modern oil did not have enough suspension (MEP vs EP) to stay in the void of the gears resulting in not enough oil cushion between the gears.
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