The "steam" from your oil cap is most likely oil vapor. Has your engine been run recently? I seem to get that more from the old engines that have been sitting a while. Excessive crankcase pressure from sludgy oil and rings and valve seals that allow lots of blowby.
I usually try some light kerosene or diesel fuel mixed with some new (but cheap) motor oil for long enough to break all of the sludge loose and then change the oil.
Then some 5w-30 conventional motor oil with a light amount of running to try to break in the rings once more. You will still have some vapor because you don't have good enough valve seals to stop it all.
Last, you need to pull the intake and the valley cover after you run the engine flush mix through it and then clean out the valley and the blow by tube/fill tube as much as possible.
Good luck!!
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