Ran my 454 Mag Velocity at the beach last weekend. Checked the oil Monday morning and saw about 1/4" at the bottom of the stick was milkshake. Oh crap water in the oil. I drained the oil and flushed the motor with fresh water. The drain oil was not as bad as I thought. While it was running I noticed the starboard side was hot, 150 on the gage. The port side was luke warm at best. Pulled the dip stick again after the flush and saw the same 1/4" of white at the bottom. I pulled the Imco Thumper manifolds off and did not notice anything cracked or bad gaskets. they did have a little scale in the drain holes. I pulled both valve covers the next day and noticed the roller rocker arms were dripping wet with condensation. The top of the heads were a bit milky as well. I opened up the drain holes in the block to let the water out and had to use a screw driver to loosen up the scale to get the water to really drain. Could the water in the oil be from the cooling system not letting the port side warm up to temperature? Trying to eliminate possibilities before I tear the heads off to inspect the head gaskets.

Yeah it's possible. I had slightly milky oil after idling a 454 a lot in cold weather and never allowing it to get up to temp. I'd take it out, run it hard and get the oil hot enough to boil the water out of it and check again. If there is no water in it, probably not a head gasket issue, if there is a lot more water in it, there is definitely an issue.

Thanks. I did do a lot of idling around the night before. I am pressure testing the manifolds as we speak.

Which 454 Mag motor ? Year ? Any salt water use ? Do you have a stock recirculating pump system or crossover system ? Stock or aftermarket valve covers ? Breathers to flame arrestor or just vent at valve covers, not connected to flame arrestor ?

Boat is new to me. Was told its the 365 Mag. Not sure what year the motor is, guessing early 2000's. First time I had it in the salt water i had the issue. Motor is cooled by pick up in the drive. Its not a closed system. It has aftermarket finned aluminum valve covers with breathers to flame arrestor.

Okay, how about a pic then ?

The engine is obviously not stock. What I'm seeing looks to be caused by condensation and the oil not getting hot enough. If I had to guess, they added a bigger oil cooler to the engine when they did the hp modifications and did not use an oil thermostat.