Why use that overpriced HD oil (synthetic or regular, it is overpriced) when a 15W-40 dual rated CI-4/SL diesel/gasoline engine oil like Shell Rotella or Chevron Delo is $6/gallon? It has NO friction modifiers to cause problems with wet clutches, which is the only reason I've ever heard to use special "motorcycle" oil. It's also got significantly more detergents and ZDDP anti-wear additives. (The API was concerned about ZDDP's effect on catalytic convertors so typical gasoline engine oils have reduced levels of ZDDP).
Lots of information on this at the http://www.bobistheoilguy.com website -- check the motorcycle lubrication forum.
Actually, the original reason I put the synthetic in was to aid in maintaining running temp a few hundred miles after I rebuilt the engine.
I orginally put the 20-50 HD in there when we built the motor I guess around this time last year. The only thing original are the cases, flywheel assembly, rocker boxes and covers and jugs. Also shaved the bottom of both jugs and punched em out but didn't stroke it.
Then my engine red light came on, two separate times, during two separate traffic jams here in Floriduh. I had about 500 to 600 miles on the rebuild when this happened. So I tossed around the idea of an oil cooler but couldn't come up with one that fit, or that I liked, or a price that fit my wallet, or one that would do what I wanted. So, in went the synthetic.
It is now time to go back to fossil oil.
I was a diesel jockey for years and my trucks lived for Rotella and Delo (sometimes both!). Being I'm connected with the local repair shop through a bloodline, I prefer HD or revtech.
HD holds a higher consistant pressure than Revtech, but Revtech runs a hair cooler because of the lower pressure.
Also, being it's August, I'm ugly and my breath stinks, I'll try some straight 50 HD until Sept or Oct. Then go back to a multi-grade.
She's putting out HP in the low 80's. Heavy bike too. 835lbs + me.
Take a look at this old write up from '94 at the listed URL below. Gives a good insight on lube properties.
http://www.nightrider.com/biketech/oiltest1.htm