I have been running hot on the water temp gauge and I will be adding an oil temp gauge as an additional measure to avoid an overheat.

What is the link between oil weight and it's ability to shed temperature?

Currently, I am running Castrol Syntec 10W-30. It has been suggested to me that it is too light and I should be running Merc 25W-40. Will this change make the motor run at lower temps?

The motor is a stock 2000 7.4L MPI.

I plan on installing the oil temp gauge and then running it with the Castrol Syntec 10W-30 and then changing to the the Merc 25W-40 to see how much of a difference it amkes.

Look for Oil the thread in tech from Hydrocruiser. In general tough to beat Mobil 1 15-50 Silver Cap from Wal Fart.

Ditto, 30w is too thin for boat engines.

I have always been told to run either straight 40 or 15-50. Valvoline 40 is what I have always used.

Royal Purple 40w is our synthetic choice, and Brad Penn is the traditional oil of choice. Carder

I highly doubt that you will see a change in water temp due to a higher oil viscosity. I would check for a blockage (sand, debris) and/or a worn impeller and housing. Saying that a heavier wt oil will give your motor better protection overall.


Darrell.

A thinner viscosity oil will absorb more heat than a heavier oil, therefore your oil sump temps will climb when changing to a lighter viscosity oil. Take more heat out of the combustion area via the oil, it has to get warmer in the sump.
When you change to a heavier oil, and the oil temps drop, less heat is being carried away by the oil so the tops of pistons, rings, exhaust valves, etc run hotter.
Changing oil does not change anything to affect combustion temps and thats where the heat comes from. What affects combustion temp is fuel ratio, load, timing, etc. not oil viscosity.
If your oil sump temps drop say 25 degrees from changing oils chances are that there wasn't 25 degrees of frictional heat in the engine and it just the oil abilit (or lack of ) to absorb and transfer heat.
Ken