What oil for a "4"?

I've been out of antique bikes for many years now, but every once in a while an old friend or customer drags me back in.
I have to do a service on a "4" and I can't remember what kind of oil to use.
Being it doesn't touch the clutch, I'm tempted to use a modern synthetic multi-grade.
Thanks in advance.

are you talking about a inline four ? if so i would say that your choice would swiftly kill that engine imho.

Hard to imagine that anything back when this thing was made is as good as a synthetic multi.
I could be wrong.
I think they call for straight 30...maybe 50...
But anything written in my books will be from 75 years ago.
So...what are the guys who actually run these things actually running?

charlie ,try asking them here

http://www.indian4club.org/

regards jib

Thank ya kindly.