Well, if you run it with the oil cap off, then the crankcase gasses will go in and out of the oil filler every time the pistons go up and down, at a higher velocity than the intake and exhaust (higher because the hole is smaller than the intake manifold or the exhaust pipe). The air in the crankcase has to go somewhere when the pistons come down, and the easiest way out is the oil filler. And there's oil splashing around, so of course it will spit a bunch of oil out in a mist and drops.
Just put the darn oil cap back on. If you're worried about blow-by, that's not the way to measure it; check the compression with a compression gauge. If the compression is OK and there aren't any new noises, then just fill the coolant back up, burp it and ride.
Last edited by Squeazel; 05-05-2017 at 10:42 AM.