My Moto Guzzi specs AGIP Rotra MP 75W-90 for the transmission and rear shaft drive. It's impossible to find, but Mobil Delvac 1 Synthetic 75W-90 is a virtual match in all the spec sheets, and they both have the MT-1 (manual transmission) rating with a seal conditioner.
There's a gallon sitting here on the shelf. It appears to work well in that Moto Guzzi, and I'm temped to try it in the Harley transmission.