Mine was puking oil too, thats what brought me to this site. Leaf cutter bees look for a specific size hole and plug it with something like mud.
Cleaned vent ( under toolbox) and problem was instantly cured.
So does that vent screw in, or is it a press fit? I tried some soft steel wire on it, got some dauber mud out, but when I twisted the fill cap off, it felt like there was still some pressure on it. Was thinking of putting a vent plug like a one way Hyd cylinder has on it, to make it dauber proof...
Screws out. I had to remove the tool box to unscrew it. Mud dobbers tend to get into the hook on it. So i end up pulling it out and take a coat hanger to both ends to loosen up the crap and then blow some compressed air through it to make sure it was clear