I think Castrol has really stepped to the plate on their Syntec Synthetic and synthetic blends. I used both in my in 930 and Audi Quattro and they have held up remarkably well. Change the oil in the Audi every 2 months and the 930 twice a year.
'85 930 Factory Special Wishes Flachbau
Werk I Zuffenhausen 3.3l/330BHP Engine with Sonderwunsch Cams, FabSpeed Headers, Kokeln IC, Twin Plugged Electromotive Crankfire, Tial Wastegate(0.8 Bar), K27 Hybrid Turbo, Ruf Twin-tip Muffler, 8:41 R&P