sounds like I'd better do another plug for the counselling session on the 12th August, Northcote Tavern after 6:30...
I can't be much help to you there, my race engine runs on Morris 40 castor oil and so I can always swap engines if need be, so my road A10 (super rocket project) will also run on it.
So not compatible with any mineral oils I could recommend.
The one thing I would say about engine oils is that firstly do you know much about the history of the build. The A10's run a sludge trap and quietly accumulate the stuff. So unless you have a nice fresh sludge free crank I would recommend a monograde (Morris do a good straight 40) like they ran back in '60. If you have a nice clean motor you can opt to run modern multigrade oils with detergents but should also have a modern return oil filter and a clean oil tank.
A10's must have clean engine oil for long life.
Also recommend cleaning the sump gauze routinely