I don't know if this has been covered or not, but I've wondered over the years if it hurts to switch up brands of oil in a motor. I've always heard that you should stick with the same brand once you start with it in a vehicle. If that's the case or not, would that also apply with synthetic? I recently got Valvoline 5w30 synthetic on sale at Oreilly's for $3.99/qt, so I got 12 qts, but if I catch some Castrol Synthetic, M1, or PP on sale, I don't know whether to load up on it as well, therefore switching up brands but staying with the same weight (5w30) in all my vehicles.....