Hello Andy!
Your question IS quite taxing, in fact. Its very hard to give a recommendation here.
I would say it strongly depends on the condition of your engine. If it is freshly rebuilt, within close tolerances, it might be okay. If the clearances in the bearings and the oil pump are rather large already and the engine is leaking oil, things will get worse with the thinner 15W. BSA stated SAE 30 for winter use, which is significantly thicker than 15. I am using 20W50 and find this okay for my rebuilt engines with bearing clearances (main and big ends) of 4/100 to 5/100mm (less than 2 thou). If you want to use the 15W50 it would be a good idea to measure oil pressure, esp. at low revs/idle when warm, just to make sure (maybe fit a gauge instead of the pressure release valve, just for checking when idling?). Oil leaks and wet sumping definitely won't become any better.