I have been using Mobil 5w-50, I must say it is a good oil though from when it had 264kw to a now 370kw, its has never had outstanding fuel econemy like other cars ive seen. So my new choice is Nulon in my upcoming service.. but cant decide whether to run 10w-40 "hitec fast flow oil" or the 15w-50 street track mix. Has anyone used both?? and can share some experiences with me? Remember I am running 15psi @ 370rwkw, though I can say I dont thrash it too much.