Formula+ is nothing more than a 50 weight mineral oil with virtually no additives.
Virgin Oil Analysis - HD Formula+
Eh, but don't fret - most (automotive) manual transmission lubes are like this (but thinner) and they never even change the fluid. What this means is that your Harley transmission is not a candidate for high-tech lubrication.
We are currently using Valvoline VR1 Racing Conventional oil 20W50 in Sportster crankcases and transmission/primaries.
You can use the Mobil 1 V-Twin 20W-50 in that shared transmission. Same for Amsoil MCV. You could probably use Rotella conventional 15W-40.
Do not use gear oil. Gear oil may not be friendly to yellow metals and your stator (copper windings) is in the primary chaincase. Just sayin'
There are a lot of things you could use in the shared transmission/primary. Think about it this way - most japanese bikes have a sump where everything shares the same oil and they typically run a xW-40 grade oil. The newer ones are spec'ing a 10W-30 oil.
Crankcase? Kinda depends on how often you want to change the oil.
Believe it or not, there's a UOA here that shows Genuine HD 360 conventional oil after 5,000 miles and it performed well. Valvoline VR1 Racing Conventional oil also does well. Oh, and you should probably stick with the manufacturer's recommendation of 20W-50