Man, I agree that this topic can make your head spin with t he more you read.  It's like the octane conversation with fuel.. I too have read a ton on these subjects, yet when I try to explain them to someone else it usually sounds more like a 6 year old trying to explain where babies come from.

I pulled this article off the sight Vintage240zracer posted.  It's really close to many others I have read explaining viscosity and oil weights.  It's worth the read just to kind of understand how your oil really works in the engine.

http://www.bobistheoilguy.com/motor-oil-101/

The only problem I have with that article is that he is taking into consideration that the clearances and operating temperatures are for an automobile. Cruising down the freeway on a Sunday tour of the wineries in the area.

Now open up the clearances, spin it six times faster and increase the oil temperatures... All the math just went out the drain plug...

When oil is put under the strain that a Jet boat motor can put unto it, you can take virtually any article written for a car and toss it...

JMHO...

BTW, the article is awesome for your family sedan. Your muti-vis is likely not doing what you think it is... here is a test, does your oil pressure stay exactly the same at idle warm or cold... Probably not.

Not saying it is doing any harm at all, just saying it is not necessary.

GT