Tohatsu 9.8 4 cycle- Milky oil?

I have a 2006 Tohatsu 9.8 4 cycle that has given me good service on my cruising sailboat with lots of use for the last 12 years. This last Sept when checking the oil I noted it was milky. I changed it a couple of times running it with a water muff between changes and the milkiness went away.
Heading south for the winter we used the engine a few hours over the period of a few weeks and found the oil was milky again. We changed the oil again a couple of times running it on a muff between changes and then took the engine out of service until I could get it home and tear it down.

Having torn it down including: removing the engine from the outboard, removing the head, the water cover on the thermostat, the exhaust mixer on the discharge and the water pump I find no "smoking gun" I was expecting to find evidence of a leaking engine to outboard base gasket or leaking head gasket but both gaskets and the seating surfaces look fine.

Both cylinders look pristine and the head looks just like it should. I can find no obvious cracks in the head or block.

The internal zinc was about half consumed and the thermostat looks pretty cruddy and the water pump impeller, while still serviceable, will be changed out.

Any ideas? Do I just put it back together with new gaskets and hope for the best? BTW, I have the shop manual.

Bright Ayes

Assuming your model is MFS9.8A3. Year is irrelevant.

In order for cooling water to make it to the oil, you would have a problem such as a bad head gasket. Severe condensation could be an issue as well. Of course, you should be running 10w-30 non-synthetic oil to minimize oil gain. Yes, if it's milky, it will require several changes to clear it all up.

I have seen instances where a brief splash/submersion caused a little water to get into the crankcase. That was on a kicker on the back of a power boat. Similar to being on a sailboat.

Yes, it's time for a new anode and a new stat. Also, change the complete wp kit -- not just the impeller. It will last longer and pump better. In cold fresh water, we do that about every 3 years, even though the Factory says to inspect/replace annually. In cruddy water, we do that more often. In sandy water, I have a customer who does his 2x per year.

If it does not get milky on muffs, but does in operation, verify that your height is correct. If you get serious splashes, it could be drawn into the crankcase.

If it were my motor, I would reassemble it and test on muffs. If no milkiness, that shows that, even under pressure (from the hose), the motor integrity is OK. If that's the case, again review how it's mounted on the boat.