Castrol synthetic or Valvoline?

I moved away from Castrol products in the early 1990's, as I found I wasn't getting the mileage I was expecting before my lifters started chattering.

Since I switched to Valvoline, I don't have that issue any more.

This suggested to me that the Castrol was breaking down sooner.

So far I've been very happy with Valvoline syn blend 5 or 10W 30, and I've also been pleased with Motorcraft syn blend in the same weight. Both are very competitively priced.