brad penn is the Best oil for these engines IMHO.
We buy it in 55 gallon drums. The word synthetic means very little and/or nothing now as far as labeling. Dino oils are called synthetic all the time once they have been manipulated hard enough. It's retarded.
I'm REALLY surprised your guy is using Castrol GTX...it's a much less expensive option, but IIRC it doesn't have nearly as strong an additive package when it comes to older engines... Considering that you can buy 5 qt's at the Walton's Store of Death for 18$ I'm a gonna say that the additive package it junk. I'd drop that out of your 993 Yesterday.
With out being able to hold a camshaft in my hand and look @ it Ever so closely knowingly I would not buy used cams. Alt, call tom @ the Classic Center give him your VIN and see what he can do. Do you have the 2 digit code of the back of your cam shafts? That helps identify what all you have as well.