In case you want to see my transaxle rebuild project, take a look at this link http://members3.boardhost.com/928s4vr/msg/35534.html
A 928 transaxle costs $13,000 new, and around $2500 to $3000 exchange "IF" you don't have major problems with it. You'll note the entire rear suspension has to come out of the car in order to remove the transaxle. Therefore, using the "best lube available" is in order. Same goes for the motor.
Right now I'm running Mobil-1 15W50 in my marine 427s, and as you can see from the prev chart I posted, it doesn't stack up too well against Amsoil. It is, however, far superior to just about anything else, and for the dollar, it's a very good product. I think Red Line makes a fine product too.
Personally, I think Amsoil makes a heck of a product. Porsche, by the way, is now actually "recommending" Mobil-1 as their preferred oil.