Stephen Barner has it right, I do believe, when he writes:
Internal hub gears take a pretty high load, as far as bicycle parts go. But the mechanism is quite similar to the innards of a quick change lathe gearbox, especially those of 20+ years ago. Straight-cut gears in a low-temp environment taking a medium load at low rpm's. I would suspect that 20 wt machine oil would work as well in a S/A hub as it does in a South Bend lathe. +++++++++++++++++++++ With fond memories of WW-II-surplus South Bends, I'd just add a couple of notes: (1) We were certainly told by the sewing machine folks that 3-in-1 leads to waxing in that machinery, and have used actual sewing machine oil instead. (2) I don't use automotive motor oil (the SAE series) usually, because they have all sorts of viscosity control and detergent and who knows what other stuff to meet the extraordinary demands of that high-load, high-temperature environment. (3) If you actually ride your S/A in very cold weather, you might want to get 10 wt. oil :-) (4) I have never tried (differential) gear oil, which is 90-wt. Might work, but that is really designed for a much more hellish load. Hey, lads & lassies, lubricating internal rear hubs is not rocket science; the original S/A patents go back to 1903, I think, so we have a century or so of experience with this...
harvey sachs
mcLean va