Question, why run a multi-grade oil in a boat? If you boat in the southwest the oil is 80/90 degrees before you even start the engine.
Each viscosity rating is for a specific temperature. The base oil will naturally loose viscosity as temperature increases. A multi-viscosity has additives, called viscosity modifiers, which are spring-like molecules which extend with temperature, increasing viscosity.
The theory is, thinner oil will flow more freely to the needed areas when the engine is cool. This can be an issue with the oil filter as excessive pressure can blow it apart. The clearances increase as the temperature increases, causing less of a restriction to oil flow. The result is that oil pressure drops. Remember, pressure is the result of resistance to flow.
I have been looking for a direct correlation between clearance and viscosity, as opposed to the trial and error method, to no avail.
This video is a great presentation illustrating oil flow relative to bearing clearance: Watch "Oil Pump Pressure vs. Flow" on YouTube
