I use Penrite "HD and Indain" oil (available in OZ) -which is a straight 50 I think in my 47 Chief and earlier Scouts - seems to work OK - a very "sticky" oil.
I once tried Castrol 2 stoke racing oil that at the time people in our club were raving about - went through everything like water - may be Ok for English bikes perhaps but definitely not for Indians.

While on oils - can anyone recommend a suitable grade for a 41 Four.

If I ever get my import application approved ( now been 10 weeks) and get the four here - first job is to fill the oil as its been drained for transportation.