My well maintained 8 year old 550EX just started expelling oil from the breather tube into the air filter box. My oil level is perfect and the machine has not been tilted or inverted. It seems that this is due either to either excessive crankcase pressure (from bad rings or head gasket) or the breather assembly is allowing oil out when it shouldn't.
You are right about the blowby. You can verify this with a compression tester; if the compression is improved by squirting a *small* amount of heavy oil into the cylinder, it's a bad ring or heavily worn cylinder. Since it started suddenly, it's a bad ring (and repairable).
The EX series seems to suffer from a lot more bad crank case breathers than other B & S push mower engines.
Simple test is to pull the breather tube off the back of the air filter
Hole your thumb over the tube.
It should only blow
If you can feel it blowing & sucking or not blowing at all it needs replacing.
