Are you using a synthetic or mineral 20w-50? It's should be okay, but a synthetic is a better choice than a mineral oil
Hi
Mobil 1 is probably okay, but it is fairly thin, so with a modified higher mileage car, it may get burned off at a fairly high rate. I would use a 5w-40 synthetic
http://www.opieoils.co.uk/c-656-5w-40.aspx
Out of those, the best ones are the Fuchs/Silkolene Pro S, Millers CFS, Motul 300V, Redline and Gulf Competition. The Motul 8100 X-Cess, Millers XFS, Fuchs Supersyn, Shell Helix and Mobil Super 3000 are good, cheaper alternatives.
Cheers
Tim