1. 75W-110 Gear oil is virtually the same viscosity as 20W-50 motor oil. Harley-Devidson Formula+ (the recommended transmission/primary chaincase lubricant) has a nearly identical viscosity.... without the temperature range.
2. Harley Davidson Formula+ is basically a mineral oil with little or no additives. Many Manual Transmission Lubes (MTLs)have a similar recipe except that MTLs are in the 20 weight range. Redline makes a "heavy" MTL that's in the 30 weight range.
3. The contents of the primary chaincase will have more effect on the shifting "feel" than what goes into the transmission. This has to do with the rate of clutch spin-down.
Amsoil MCV 20W-50 is absolutely fine for all three holes. Or, Mobil 1 V-Twin 20W-50. They're virtually identical. Many like this setup because of the convenience factor.
We like Mobil Delvac 15W-40 in the primary chaincases for the best shift "feel". Of course - you could use Rotella or... or.... or....
Run what ya want in the transmissions. Most anything will give excellent service and it doesn't need to be gear oil. In fact, doesn't Amsoil recomment MCV and then gear oil as their SECOND choice for Harley transmissions?
.... but don't run gear oil in the primary chaincase. GL-5 lubes are sometimes not kind to yellow metals (copper, brass, etc) and your stator is located behind the compensating sprocket at the front of the chaincase (copper windings).
We now return you to your regularly-scheduled program.