Briggs and Stratton 18.5 OHV pushing oil out the exhaust pipe.

[Bat57]

Oil gets into the engine from 3 sources
1) past the rings
2) down the intake valve guide
3) through the carb via the breather

The driving force for this excessive pressure in the crankcase .
So in your case the breather is restricting the flow of crankcase gasses and the oil is blowing out the breather then into the engine via the carb

Thanks I will check this out.