I use Penrite oils (one of the mono-grade classic ones like Shelsey Medium) and Regular 91 pump petrol in all my old side-valve bikes. There might be better, but I have no trouble. If the motor is new an egg-cup of oil in a tank of petrol probably won't hurt.
If you've thoroughly cleaned out the insides of your motor, any motor oil will likely be fine for a lifetime of occasional vintage rallying. Popular wisdom is to avoid modern oils with detergents in old, sludgy engines.
Good luck,
Leon