Can someone clarify something for me here... I buy the 1L castrol 20W50 Mineral Oil, as told to do so by CWs. It costs me £5.49 a litre. Can I use any 20W50 mineral oil and can I top up with a different brand than the one I have in. For example, Halfords do a Classic car Oli which is 20w50 mineral for about £12 for 5 litres - can I just use that?