While I already put in my vote for Quaker State several pages back, I just wanted to reflect on a non-scientific observation.
When doing a back to back pourings of Valvoline and QS, the Quaker State seems to have a healthier, more robust look to it. Valvoline seems to have the appearance of... dirty dishwater. lol
I know this is technically meaningless. I also know both oils are very, very good. In fact, I have used Valvoline many more times than QS. But, for now, I'm sticking to my original choice.