If you ever notice gas in your mower’s air filter, or worse, leaking out of the filter’s cover: don’t start the engine without servicing the carburetor and changing the oil! It’s an almost guaranteed way to cause engine damage.
For most consumer mowers that use bowl-type carburetors, this is usually caused by a stuck needle valve, junk in the valve’s seat, or a damaged float. When this happens, gas is flowing right past the valve and filling the carb (which you’re seeing overflow into the air filter). From there, it goes straight into the engine cylinder. That gas will eventually leak past the cylinder rings and mix in the crankcase oil. And that diluted oil can damage (or completely seize) an engine rather quickly!
Shorter version: gas has mixed in with (and severely diluted) your engine oil. Chafing is not a good thing…
Here’s a pretty bad example. When we tried to start this Poulan Pro model (Kohler engine), gas gushed out of the air filter cover. Inspecting the carb found a needle valve stuck wide open. After rebuilding it, we drained the “oil”. It’s a little hard to see in the picture, but the oil is super thin, “milky”, and bubbly. We were able to start it after the fixes (with fresh oil), but unfortunately the previous owner appears to have ran it this way for a bit. The engine isn’t seized, but it’s definitely damaged and rattling pretty hard.