Amsoil 75W90 is rated for the 944 and it made a huge difference in my car. Mine has 165,000 on the odometer and was making some noise from the rear end, and I switched to the Amsoil, which has a superb rating. It quieted the tranny down, and made it so much easier to shift when cold. Very very good stuff. Here is a photo and direct link. Take a look at the specs, I doubt if you will find anything that can surpass it.
http://www.amsoil.com/products/tgr.html
I looked at the Mobil-1 gear lube, and it does not test out as well as this stuff does, by the way. Mobil also costs less, and I assume you get what you pay for. The Amsoil is rated from GL-2 through GL-5.
In dino oil, the sulfur would react with the yellow metals in the transmission and cause corrosion if it got hot enough. The Amsoil runs so much cooler due to low friction, etc. Because it made such a difference in the ole '84, I've made a point to use it in the other cars too. I highly recommend this product to anyone with a 944.
P