B31 is on Castrol XXL (straight 40) 'cos I 've got gallons of the stuff. The A7 is also currently on XXL but is usually on Morris's 20-50. The A10 is on Morris's 20-50.
Really it's a question of good quality monograde oil or good quality multigrade. My general rule of thumb is monograde for roller bearing big ends, multigrade for plain big ends.
There is massive discussion elsewhere on the levels of ZDDP additive in classic oils against the reduced levels in current oils which have to avoid contaminating exhaust catalysts.
The bottom line is that any reputable oil changed reasonably regularly will do the job.
