From what i've read, and in my personal experience, as far as conventional oils go, Castrol and Valvoline are very similar in additives and performace, both being among the best. Mobil conventional products are decent formulations, typically using a higher moly count than some others. Havoline recently went through some changes in their formulation and the verdict is still out on it. Pennzoil seems to continue to turn in good results for people in many applications. In used oil testing Pennzoils cousin QuakerState also seems to be turning in some very good numbers.
Any differences however seem to be very small. All SM rated oils seem to be usable in most applications that call for them. Even store brand oils seem to do a good job. It's harder and harder to find a 'bad' oil these days.
Having said that ive also found that certain cars seem to "like" certain oils better than others. I'm assuming this has to do with finding the viscosity that is just right for your car. Some 5w30's are thicker than others, thiner, likewise with 10w30, etc...