80w90
The newer differentials take a synthetic oil but I do not know which type
80/90w gear oil
For a 1987 Suzuki Samurai axle, it's recommended to use 80W-90 gear oil, which is suitable for most differentials. If the vehicle is operated in extreme conditions, a synthetic 75W-90 gear oil may also be used for improved performance and protection. Always check the owner's manual for specific recommendations and ensure the oil meets API GL-5 specifications.
Gear oil (80W90 or similar) is used in the transfer case and differentials.
Sae 10w/30 "se" or "sf" motor oil
You should be running 10W-30, In a 300 / 302 / 351.
The service manual says.... The transfer case near left foot uses engine oil - GN4 4-Stroke Motor Oil. 10w40 or 10w30 The differential's uses Hypoid SAE #80
10w30
10w40
80W90.
no you should use gear oil
Gear oil is actually an oil as used in transmissions and differentials. grease is thick sticky stuff (possibly like the one that reckons oil is grease) that wouldn't be much good in a gearbox as it would be unable to lubricate all the parts