I got my oil changed at walmart and the employees commented that its the wosrt oil ever and that's why its on sale for the wlamart 24 dollar oil changed. They notably gringed on the thought of putting it in a diesel. My gfs dad is ase certified (if that means much) and agreed with empolyees opinions. So I was just wondeing why? Also is there really an appreciatble differences in performance say between major brands? Say castrol vs valvoline and their respective oil types (high mileage vs high mileage or syntethic vs synthetic)?