New 4matic owner - what oil in transfer case?

[leylandp38]

I have just acquired a 1988 300TE 4Matic and I want to check all the fluids. Can someone advise what oil the transfer case takes? I'm guessing its hypoid gear oil most likely?

Is it a good idea to change the transfer case oil over periodically or is it something you should just top up?

Any 4matic tips/tricks gratefully accepted.