But what if your or BSA or Triumph or Atomic-4 don't
have a full flow filter? Well then get nondetergent multigrades from Morris
in England.

I use Castrol 5W-50 synthetic in my A/4. It makes for much less cranking temp
until oil pressure comes up plus ten seconds more (I set the throttle so the
engine won't start to prelube), and change the oil frequently.

Synthetic oil flows at 300*+ plus while mineral oil does not. This means my
exhaust valves last loooooooooooooots longer. (The A/4, like most engines
designed prior to about 1970, used the lead in the fuel to lube the valves and
valve stems.)