Bought the motor already put together. I think it's the headers loading up with water. There was water poring from the little holes in the headers.
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Headers are supposed to do that.
If any water gets into the cylinders through your exhaust ports, muddy oil is gonna be the least of your problems.
Most common "milk shake" problem is not enough silicone around the water passage at the intake manifold.
Second I've seen is the head gasket.
REGULATE WATER PRESSURE GOING INTO BLOCK!
-beerjet-