Hi, Can anyone out there tell me what oil to use to fill/top upthe diif on my '86 944[&:]

 

I use redline

 

RedLine is good. Your best bet is tocheck the owner's manual for recommended weights and minimum quality requirements. The same stuff used in the 5-speed goes right into the differential (they're not internally separated.) You may or may not have a limited slip gearbox, which might make a difference in the type of product you select--typically designated "for use in transmissions with limited slip differentials.