Additives for ZF fluid change

All rear end lubes are for hypoid type gears since about 1920. The variances are for so-called 'positraction-type' limited-slip differentials. The ZF transaxle is in fact a 'posi-type' limited slip differential with spring-loaded slip-clutches on each rear axle. I purchase and use 75W90 posi-type lube and it works well for me. Some rear end lubes will say "for posi-type diffs" while others do not. If you decide for whatever reason to use lube not specifically for clutch-type differentials (e.g- posi-type), I suggest the expensive limited slip ZF transaxle will last longer with some posi-additive in there. Regardless, the clutch plates will eventually all wear out leaving us with what is called an 'open-type' differential.

Thanks, Jack. I understand exactly what you've explained. I appreciate your taking the time to give us all the benefit of your extensive knowledge. One further question: What brand of fluid do you use?

Michael

Hmmmm.... good question. It's been 13 yrs since I dug into my ZF, but I believe it was simply generic 'posi-compatible' 75W90 gear lube from a neighborhood auto parts store. If you change the fluid and the ZF starts shuddering in tight turns, the stuff you added does NOT have enough slip-agent in it so the posi-clutches in the differential are stick-slipping. Add a few ounces of GM's posi-additive, and the problem will go away in a few miles.