GL-5 gear oil has a very pungent odor from the additives that are required to meet the extreme pressure GL-5 specification. If you spill any on your clothes it is almost impossible to get the smell out. Make sure whatever you use in the axle is GL-5 rated. A 85W-90 viscosity may be called out, but these are not common in North American, and common 80W-90 GL-5 is just fine.

Transmissions and transaxles without hypoid gear sets usually only require a GL-4, but hypoid axles will not last with GL-4.

Most manufacturers consider open differential axles "lubricated for life" other than maybe a break-in change, and only recommend periodic level checks, adding oil as required, however, if the axle is a clutch type limited slip, it is a good idea to change the oil every 30-60K miles to purge the clutch wear debris, and, depending on the gear oil used, clutch type LSDs require a "friction modifier" additive to prevent clutch chatter and this additive degrades with time. A few available axle oils have the additive. Most don't. Using an oil with the friction modifier additive in an open differential will have no effect, but is a waste of money.

If you have an open differential and are a maintenance zealot, changing the axle oil every 60-120K is reasonable and there is no time requirement.

If the oil is foamed up, I wonder if water has entered the axle, which could certainly happen if the car was subject to flood waters, or it may have not been a correct GL-5 rated oil.

Duke