In years past, when I raced an XR600R (air cooled), we found Mobil 1 15-50 (was red cap back then) to be the best oil. The engines held up very well with that oil. Contrasted with some "motorcycle specific" oils at the time, Mobil 1 was a SUPERIOR oil. The M1 eliminated some piston ring and valve tappet/camshaft problems that were becoming expensive.
Mobil 1 MX4t is a great product. Use the proper 10-40 for your bike. FYI, MX4t has high levels of ZDDP (anti wear) (you may know that ZDDP has been reduced in most automotive oils today). MX4t is also a full Group 4 synthetic, able to handle the heat of an air cooled engine without breaking down.
I see MX4t at the local WalMart from time to time. WalMart is cheap by comparison to the bike shops.
The M1 20-50 V-twin oil is also a very good oil. If you must run thicker oil. This is among the very best! I ran it in my turbo miata (33 pounds boost!) with excellent results.
Chris