Castrol Racing oil 10W/60

My father used Castrol R 10W - 60 for years in his 1993 HSV Clubsport 5.0L. He found the oil made the car feel a bit sluggish so recently, he changed to Castrol R 0W - 40 and he believes the car runs much better. He doesn't drive the car hard so there should be no issue No doubt the 10W - 60 is a premium oil though, just relatively high viscosity.
Good? Oh, yeah... I've been using it for the past 10k mi. (2 intervals) in my 1994 BMW. The car has never run better. I haven't done an analysis yet because I'm waiting for the crap to be slowly flushed out from years of 5-30 use...but, I will do one eventually - once I feel safe to complete 10k on a single interval. Will also be intersting to see how it does this winter...