Mobile 1 in all 3 holes. Harley recommends 20w-50 in all 3 holes and it was designed that way. No more old school that only GEAR oil should be used for transmission. IMO.
Actually, Harley still uses and recommends their own gear oil in the trans and primary on everything other than the CVO's. It's called Formula+ I believe. The one lube in all three holes is something they started with the CVO's and only with the SYN3 oil.
BTW, a 50 weight motor oil is about the same actual viscosity as a 90 weight gear oil. They have different rating systems so the numbers don't match, but the actual viscosity is almost identical. The differences between gear oils and motor oils are a lot more involved than just viscosity ratings, and while you can "get by" with a motor oil that has a few extra additives for the gearbox, a properly formulated gear oil will still be superior over the long haul. Some of the best antiwear additives cannot be added to motor oil due to EPA regulations (certain things that can damage or destroy catalysts, for instance), and others are not compatible with the high moisture environment in an engine oil system due to rapid formation of acids. So the stuff that SYN3 and Amsoil and Mobil 1 put in their oil that's advertised as good for all 3 holes doesn't necessarily contain the very best extreme pressure additives available. For many/most it probably wouldn't make a significant difference in the life of the gearbox, since most don't put enough miles on a Harley to wear out the gears. But there is a difference, and there is a reason that both products exist. Old school has nothing to do with it.
Let me ask a hypothetical question. Would you put the same 5w20 motor oil you have in your truck engine in the rear axle and the transmission? Do you wonder why the auto manufacturer's don't use one fluid in all holes? It would make life in the assembly plants and dealerships much simpler and reduce costs if they only had to stock and use one fluid for everything. And yet they don't do that; wonder why.
Jerry