I just received my car back from its first dealer service and saw I had been charged £60 for 3 litres of Castrol magnatec 5w30 c2 oil. I showed how I could but the same grade myself for less than half of that price, to be told that the garage uses 'professional' grade oil, unavailable to a rube like me. Is there any difference in performance between professional grade and consumer grade? If so, is it really worth 100% premium?