I'm fighting a foaming issue now.
My oil is NOT cross contaminated. We're a nuclear unit, we have VERY good records. Plus, I am running every test under the sun on it.
All of the standard tests come back acceptable.
Yet, I have foam. Not to the level of seeing vibration issues, but it's there.
I'd be interested in what type of oil you are running. Have you recently changed to a group II base stock? Maybe without even knowing it?
I'm seeing very, very slight foam levels in other reservoirs too. Mostly running R&O oils.