If you're curious take one hour to read the technical data sheets published for the various brands of oil:
- Multi grade mineral oils were blended to provide a stable oil film at operating temperature and reduce drag during cold starts.
- The average viscosity of a mineral multi grade is higher than what you get with a single grade (40 vs 10W40).
- Modern engines are designed to operate with an oil viscosity equivalent to SAE-40.
- Semi and fully synthetic lubricants provide even better flow when cold and a more stable viscosity when warm; in addition they can't absorb water.
So you want to have more than the magic number '40' to be on the safe side for the compensator, cams and piston rings. The gearbox is also designed to use the same oil weight but will remain free of debris for a longer period; here the synthetic oil plays a better role.
You should be able to correlate all your observations (valve train noise, pressure changes, etc...) with the viscosities provided by the oil makes you mention. Before I decided to use Motul, I searched "HARLEY APPROVED" oil for eventual warranty issues and I'm quite happy with what I use.
Last edited by Expat1; 08-28-2012 at 04:07 AM.