I used what ZF doc recommended. Its a fully synthetic motor oil made by castrol and imported into the states by bmw. They use it in there M5. Castrol TWS 10w60. You will not find it at any of the auto parts chains. Seems to be a bmw dealer only oil. I pd around 5.00 a qt. and you will need 2 qts. (2.2) to be exact.
Shortly after I got my 90 I changed the Trans oil. I don't know if it had ever been changed. I just put in Mobil 1 10-30 , it markedly improved the shift.It seemed as though it slowly got better for about a 1000 miles. Next time i will make the effort to get the BMW oil.
Ok. After a lot of research, cussing and discussing, (many thanks to many people who expressed their considered opinions on the subject) here's the bottom line.
GM had Pennzoil manufacture the lube they (GM) recommend for the ZF gearbox.
The GM part number WAS 12345349. GM no longer sells this product.
The Pennzoil product is:
Pennzoil Synchromesh Manual Transmission Fluid. It comes in a 1 quart bottle, familiar yellow Pennzoil packaging.
It's at Auto Zone, on the shelf.
I do hope this helps someone,