I use Valvoline Racing oil 20w50(mineral), or rather its glooped in when serviced. A load of other alternatives will be given here over time. Try a search on here and engines. It crops up a lot.
If you change it regularly, you can go for a cheaper oil, Morris Lubricants are good and they do a synthetic 15/40 at a good price. Valvoline is very good, a bit more expensive, then you have Castrol, Shell and Mobil, which are expensive!