I have always used Castrol. It used to be GP50, then it was changed to Formula R. Always 20W50. While dyno tests have generally shown slight power gains with lighter oils, I have found that the
oil has saved me from costly repairs on a couple of occasions.
1. The 460 in my Fairmont had massive
oil surge at 6000+ rpm while I was braking from VERY HIGH speed at the drags. I was at 0
oil pressure for about 6 seconds. The bearings were inspected and shown to be as good as brand new.
2. I don't have an oil cooler or oil temp gauge in the Cobra. Before I had set up the Motec triggered idiot lights on the car, I took the car to Qld Raceway. The oil temp was over 260 degrees F, and oil pressure was unaffected. I was sure that I would have hurt bearings when I looked at the datalogging, but again - good as new !