I have a 2000 502 MPI that has water in the oil. The risers and manifolds look ok but are original with 153 hours on it. Compression test was good. I thought it was possibly just condensation from the winter. I changed the oil and ran the boat in the water for a hour to find the oil milky again. The motor was 100% winterized. Changed the oil again and re tested same result. Is the intake manifold a potential problem on this motor?

Could be intake/intake gaskets. Did the motor come up to temp? I've seen stuck tstats do the same thing in colder weather/water.

Yes replaced the T-stat with new. Motor was running at 170 deg.

[QUOTE=TYPHOON;4682230 Is the intake manifold a potential problem on this motor?[/QUOTE]

I have seen it discussed here in the past that if you're in salt water and you're not FWC the intake manifolds can have a real problem . Something to do, I think , with salt deposits accumulating in parts of the manifold due to insufficient circulation in some areas.
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Not saying that this is your problem though .

No I did not change the filter when I changed the oil the second time. I never pressurized stock manifolds but thinking its the same as a header?

My opinion is the intake gaskets. While you are in there and its not obvious the gaskets are bad, look in the lifter valley for rusty cracks. Hopefully just the gaskets

Seems pretty strange after coming out of storage. Cold storage I presume. How was it winterized & by who?

Intake gaskets or intake itself. The intake is probably corroded at the passages. Common issue in salt water. My intakes have been repaired because of this with less hours.

We did at my shop.