It would be hard to beat Shell Rotella 30 weight. Definitely don't use one of the water-thin synthetics like 5W-30 or something similar. These engines need some cushioning between the parts, since the clearances are several times what's found in modern engines that run super-thin oils. If you want to have a little cold weather cranking help, try 15W-40 in the winter months.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!