I used royal purple for both my transfer case and drive units. I do not remember right off the top of my head what they spec, but you should be able to find that around here. I know the drive units use gear oil but I do not remember if it is 75w90 or not.

As far as how you actually do it, there is a how to on the home page.

Key points are:

1. drive the car first to warm everything up
2. make sure you can remove the fill plugs before you pull the drain plugs 3. it helps a lot to have a little $3 pump/hose unit that screws onto a bottle of lube to pump it into the xfer case or drive unit.

EDIT: Remember you have a drive unit on both ends - so you have to do this 3 times; two drive units, fore and aft and the transfer case.

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2001 E53 3.0 5pd