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EDIT: I was unhappy with my first reply and researched the matter a bit more...:D

My owner's manual only specify an Automatic Transmission Fluid, as you say, the BMW friend will know. I've actually never even checked the oil level in the gear box of my E32. The car has around 250k miles on the clock and even the synchromeshes still work.

Based on the age of the car my guess would be to use Dexron II. I seem to remember that Dexron ATF was originally developed by GM to use in their old two-speed automatic trannies, Dynaglide, etc, starting with the 1952 (53?) top Buick models. It was later used in many automatic boxes, e.g from Borg Warner, whereas Ford had (have?) their own brands, starting with type 'A'.

Dexron II was used in both automatic and manual boxes until sometime in the 1990s, when a new version, Dexron III, came on the market. It is backward compatible. Newer Dexron types, which are fully synthetic, have arrived since then.

So, Dexron II is a mineral oil, not a synthetic oil. As a safe rule of thumb do not mix the two. While this is in general true it appears that it is safe to mix newer Dexron types into older ones, at least if their names only contains 'Dexron' and a Latin number - special types have more letters attached.

Hence, if you want to top up and are sure the original oil is still in there, a Dexron II compatible oil should be safe. If you want to flush the box and stick new oil into it is completely safe to use synthetic ATFs, Dexron IV compatible e.g. D4 from RedLine or Synthetic 1 from Mobil - or even MTF-LT-1 if you have money to burn.

So: top up with any Dexron II compatible oil - flush and refill with a Dexron IV compatible oil.

I would like to stress that this response is based on my general understanding of these fluids and their application, not on experience with this particular type of gear box - never even looked at mine :).
 
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