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Hi, my name is Kerry
The gray foam is water and engine oil mixed into a froth.
When fuel burns, either gasoline or diesel fuel, it produces a small amount of water which goes out with the exhaust.
Every engine has a small amount of blow-by which is combustion gasses getting past the rings and into the crankcase.
Along with the combustion gasses is a trace of water.
The heat from the engine will evaporate the water and it will go out the crankcase vent and condense on the inside of the cap.
If it's excessive, you may have a problem developing, but it could just be that your engine has a few hours on it and gets a fair bit of blow-by.
If it's running fine and the oil level never goes up or gets milky, you probably have nothing to worry about.
I would wipe the cap clean and see how long it takes to get that way again.
If you have black oil on the dipstick, you have no water in the engine oil.
If you have some creamy engine oil from an internal leak, then you fix the leak and change the oil,
it will still be a little creamy but the water would eventually steam off and out the crankcase vent.
Yes, creamy oil would turn black again as the water boiled off.
It's probably just moisture from exhaust gasses getting into the crankcase.