Ive used loads of oils and never notice the difference. It was Halfords 10w/40 that went in my Valver last time as it was half price! Not the best oil, but Id say that changing it frequently is more important than a little variation in the quality of oil. Besides, I wont buy a product from Exxon-Mobil on principle (most sunken oil tankers belong to them and they kill innocent people to get at oil reserves), so it rules out the supposedly superior Mobil1. I change oil when it starts to go brown/black (about 3,000 miles). I hear some people say that the filter works better the older it is, but Im wary so spend a fiver on a new one every time too.