I'm using the Castrol product now, but I'm going to go to GM at the next oil change. Check out www.zfdoc.com. Bill Boudreau is the expert on ZF's and quite willing to share his experiences and knowledge.
I have put "other than G.M. 5W30 trans lube" in ours before with bad results. Either too noisy, or hard to shift and too tight.
I suggest sticking with the G.M. fluid right from the parts counter at G.M. MAKE SURE THEY GIVE YOU THE RIGHT STUFF! You will see right on the label that it says "5W30 transaxle/gear fluid". If it says anything else, don't buy it. My G.M. guy tried to sell me the wrong stuff just last week. I caught it as I was walking out the door reading the label. It appears the new style lube is a synthetic blend and even says that on the label.
I put a 80W 90 style blend in our 89 model ONE time. It was fine when it was nice and hot outstide and the car was at full temp. However, in the morning it would hardly go into gear, and it took a long time to get smooth during the winter months.