white foamy crap on the inside of oil cap
Water or more likely coolant mixing with the oil.
Usually not a good thing to see.
This is one of the reasons short trips are hard on a vehicle. If you make a lot of short trips in cold weather, it is most likely condensation that isn’t boiled off because the engine doesn’t get warm enough long enough. Clean it off and check weekly. Also check the coolant level, just in case.
That’s the coolest place on most engines, and the first place to cool down enough to have moisture condense on it. It’s a signal to check your coolant level because if that’s going down, it could mean a head gasket leak is letting water/antifreeze into the oil. More often than not there is no such problem.
I suppose I was being a bit of a negative Nancy when I said “most likely coolant”.
It certainly could just be condensation. I would probably go ahead and have an oil change just to make sure the rest of the oil in the car isnt milky.
If you are the kind of person who only makes short trips it s a good idea to take your car out for an extended drive once a week (preferably on the highway) (not in stop and go traffic) and let the engine come up to temp for about a half hour or so to help evaporate the water that is or may be condensing in the engine.