LESCO Kawasaki FB460V Boiling Oil

I have a Lesco 48" walk behind with a Kawasaki FB460V engine. Engine runs fine for about 10 minutes, then the engine just loses power, starts smoking, and then oil comes out of the dipstick tube. I thought I was getting fuel in the crank case, so I drained the oil, and the oil comes out like water boiling in the pan. Once it cools, it looks like oil again, so I don't think it's gas in teh engine. I also have a 1967 Deere 112, more as a toy than anything, but I started cutting with it and it did the exact same thing. 10 minutes runs fine, then smoke, then oil boiling out of it.

What have a broken in these things?

Thanks.