It sounds like a inner primary seal infamous for leaking only when your riding or when the bike is nice and warmed up And yes it will blow back towards the rear frame and gets all over the belt too . Unless you really know what your doing then have a pro fix it . The first couple times I did it myself I didn't install it properly because it's real easy to pinch the seal when your installing it . I finally bought it to a dealer and paid thru the nose for labor . Even the mechanic said its a real tricky install . If you don't get it just right then you'll be doing it again. At least w the dealer ,if they mess it up they will redo it at no charge good luck ,

O/k I will assume it is a EVO (1995) not a Twin Cam (2005). If the leak is at the crankcase lower rear (below barrell) it is more than likely the crankcase itself is leaking. Will need a teardown to repair. It could also be the rear cylinder base gasket leaking. Oil will flow down the back of the crankcase when running.