inboards typically run 140 to160F thermostats, so the oil does not get as hot as in cars and trucks with 200 degree thermostats. Lighter oil will have more throw off from rods to cool pistons and dissipate heat from bearings. I don't think you will see an issue.
Don't confuse water temps with oil temps. the 160 Thermostat is to meet coast guard regulations. The oil on these engines runs far hotter than in cars due to the load. It would be like pulling a 400o lb trailer with your truck at wide open throttle all day long
True = there is no correlation of oil and water temperatures.
False = the 160 Thermostat is to meet coast guard regulations.
Marine engines have 160 degree thermostats to prevent salt from crystalizing inside of the engine. 160 degrees is that magic number where this can start to occur.
(this is from a site discussing the benefits of closed cooling in marine engines)
Besides exposure to corrosive materials in the water, raw-water cooled engines suffered from another major drawback. They had a thermostat, just like all engines, but it was regulated at 145-150° F. This was done to minimize the possibility that salt in the salt water would separate out and crystallize inside the engine’s cooling passages, with 160°F being the critical turning point for this to occur. "