took the boat out first time couple of weeks ago had water in the oil. Found leaky intake gasket,fixed, ran 3 oil changes worth of cheap oil through engine to flush system. When oil came out clean,put the good stuff back in (mobil 1 15-50 ) but still a cheapy filter because I knew I would change it one more time when I got home just to be sure all was tight. Problem this weekend was oil temps kept on climbing. Normally temp runs about 220-230 upto 250+ on a hard run, but quickly goes back down when pulling back to cruise speed. Never went above 4K rpm this weekend but temp climbed to 260+ and only dropped slowly to 230 at idle. Water temp showed a constant 155-165 at all times during this. checked oil cooler all is clear & no collapsed hoses that I could find making me think that the oil thermostat is stuck and not flowing oil through. Am I going in the right direction with this,or are there any other suggestions out there? BTW engine is a Merc 465 HP, NA, Gen V type.

That engine is supposed to have a 140 deg. water thermostat. It should be within about 5 deg. of that. Have you changed your impellor recently. High water temps and low water flow thru the oil cooler can cause high oil temps.

New sea water pump kit was job one when it came out of the barn this spring. Water temp has always run the same temp. year in and year out. just took out the oil thermostat and heated up the element and the small plunger seemed to move freely out when heat started to reach about 205 (IR gun reading),then as it cooled the spring force pushed it back in. Now I'm leaning to the automotive filter I had installed.will put the Merc racing filter in this weekend and see what happens, but keep the suggestions coming, I'm far from an expert on these things, Thanks.

I think you're on the right track when you suspect and oil thermostat sticking, if you have one, but I would also just check the oil system becuase if you have blockage or restriction in that system, the oil bypass is going to open all the time and not let oil go to the oil cooler on most systems and thats going to cause real hot oil as you are expierencing. Let all know what you finally discover?

Best Regards,
Ray @ Raylar

Just an update, After reinstalling oil thermostat assembly with no obvious problems went out this weekend with the merc racing filter installed and oil temp was back to normal. Temps ran between 225-230 depending on how hard I was running,note to self..Never run an automotive filter, even if just for testing purposes. thanks to all for the suggestions !! all is good with the world again.

I've found that even small amounts of milky oil in the filter will cause it to reduce flow to the point that the by pass valve is not allowing the oil to flow to the filter and cooler resulting in high oil temps.