Can leaking riser gaskets allow water to leak into the oil system. If so how would the water get through the heads?
Pulled motors last winter to install new valve springs at 200 hours. While the motors were out heads were pulled and pans dropped for inspection. All gaskets were replaced. After river testing I found water in the oil in the starboard engine. Oil level on dip stick was barely above the full mark, which might not be accurate because I have seen different levels with the boat on the trailer and in the water. The oil is definitely watered down and have never had an issue with condensation build up. There was no sign of steam coming out of exhaust. I pulled the plugs and all of them look as I would expect, none appear to be steam cleaned. Oil cooler has been pulled with a visual inspection and a mickey mouse home spun pressure test. So mickey, I think should have it properly tested. I went to check the riser gaskets and discovered the nuts were not tight on the studs. After finding these nuts were loose I read online you're supposed to retorque the nuts on the aluminum EMI risers after some run time. How the H#ll do you get a torque wrench on the recessed ones? I tightened them good and tight which will have to do. If there is anyway water can get into the oil pan from a riser I will replace riser gaskets and test. The oil with the water in it was drained immediately from the motor and refilled with a quart of diesel and fresh oil. After the plugs were pulled I spun motor three different times for about 30 seconds. Next I plan on pulling the intake plenum to inspect intake gasket. Any additional advice would be appreciated.

Any ideas on how much water? Was it milkshake?

Leaking risers can cause reversion. With your risers being loose, that could have been a factor but you say your plugs are not steam-cleaned so probably not that.

I’m thinking it could be the intake. My Merlin intakes would not seal properly at the water jacket with a FelPro gasket. I would verify that you have good gasket coverage around the water ports. I used a Mr. Gasket setup and had no problems on the test fire with closed cooling.

Yes, the oil looked like a milkshake. I could see the water separate from the oil on the dipstick when I pulled it to check it though. I’d guess I gained about 1/8th inch over the full mark with water. FelPro gaskets were used on both motors. The starboard engine riser nut’s were considerably looser than the port motor’s. Port motor shows no sign of any moisture.
Sounds like it’d it would be worth the time to pull the plenum and inspect -thank you

Yes, exhaust risers leaking water internally can cause water in oil. Even exhaust reversion can cause oil to get water in it.

Yes, intake manifold sealing can cause it too.

So can other things.

Attack what is, or shat you think is, a problem area first,

It's been discussed on here elsewhere that the 502s use dissimilar metals in their build , ie; iron blocks with alloy intake manifolds. Some people (even in freshwater but especially in salt without a closed cooling system) have had corrosion pocket in areas of the intake manifold and then rotting through resulting in water in the oil .

If all the plugs look good I would take a good look at the intake manifold gasket .

Either Intake gasket,,or head gasket

Intake removal is next up despite the wife wanting to get rid of it as is. She’s had enough of my time spent with this boat.

Turns out intake manifold gasket was leaking. Motor ran good on the hose after replacing gasket. River test, couple more oil & filer changes and it should be time to burn a 11 month old tank full of non-ethanol.

Glad you got it figured out. Be careful with that old fuel!