I left my oil filler cap sitting nicely beside the fill a year or two ago. I drove about 100 highway miles before realizing it, but my oil level was normal and I saw no splatters of oil in the engine compartment when I checked. Just leaving the cap off doesn’t necessarily lead to oil on the engine. I’m sure it is possible, though.
And yes, Julio, you can mix different brands of oil. You can even mix different weights if you need or want to.