I've always thought there was enough crankcase pressure in most engines to slowly but surely blow oil out of the oil fill if the cap was missing. It's happened on my Chevy 350 CID where I accidently left off the oil fill cap, Julio. I lost 3 Qts over 100 miles. Over the years I've seen what I thought were engines in good condition that had lost quite a bit of oil over say, a period of a few days that had the cap
missing for whatever reason. Recently in our shop a mechanic left the dipstick out and about a week later the car came back with oil spray around the engine compartment area near the dipstick tube but only a minimal amount of oil was lost. On the other hand Docnic and mountainbike are quite knowledgable so I could be wrong. I guess the crux of it is; if the cap actually blew off you've got a real problem
like they say. Is there evidence of damage from the cap blowing off? I would think there would have to be.
If you left the cap off, one would think that the open oil filler would be a nice place for oil to exit during say, acelleration when blowby gases are high and manifold vacuum, which the PCV system utilizes, is low. You should have your crankcase pressure checked at a garage you trust ASAP. OK to mix different brands of oil nowadays. Let us know what happens, Julio.