How do you define spewing? Is it a big gush? or just a little? Are you sure it's engine oil? Different color than primary. There's only two things that I can thing of right away to make engine oil get into the primary. 1, bad crank seal. 2, blown seal between the crankcase and tranny where the two case halves meet. Both of these are worsend by bad venting, or bad breather gear timing. A buddy of mine years ago had the same concern when he filled his primary up to the inspection cover rather than just the screw towards the bottom of the primary. I would make sure your tranny is vented, your engine breather tube isn't plugged or too small, and you breather gear is timed right before I took the primary off. Was your motor recently apart? Or have you rode it for years and it just started doing this. Worse case, it's a blown case towards the back at the wall between the tranny and crankcase but i doubt that's it. Could be leaking there though. What year sportster?