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
Is that temp, or press?
I wonder what results would have been if they'd tested some 15W-40 diesel engine oils. I believe those are more shear-stable (withstanding the effects of the gears--which tend to make an oil thinner by shearing the molecules apart--which could result in lower pressure at operating temperature).
Also, those tests were done back when API SG was the standard..we are now on API SL (very soon to be API SM). API SL requires big improvements in the base stocks over what API SH/SJ required. For example, a typical 5W-30 of API SG vintage could have been formulated entirely with Group I basestocks. Now, an API SL 5W-30 requires the use of hydrocracked Group II or Group II+ basestocks in order to meet the volatility requirements.
When API SL was proposed, the oil companies said that they would have to use synthetic (Group III or Group IV/V basestocks) to meet the requirements, but they were able to do it with conventional oil.