While it could be leaking intake valve or a timing issue, my first guess would be your air filter is dirty - so the engine is running very rich (not enough air for the amount of fuel it puts into the engine, so excess fuel is getting into the oil and thinning it out.
Another possibility - an ignition problem so sometimes the engine is missing, putting raw gas through the cylinder, some of which gets past the rings. Check the plug wire metal ring (up inside the rubber boot) is not broken, and that it presses tightly onto the top of the plug.
A cracked or broken plug or crudded up plug can also do this - pull the plug and check for oiling, wrong gap, burnt and cracked ceramic round the center electrode, etc.
Less common cause - broken/shorting plug wire, failing ignition system, automatic choke or governor wavering or failing, loose ignition impulse block (armature) causing improper distance from flywheel magnet, worn/broken piston ring(s).
You can find Youtube videos and blogs and such on fixing any of these problems - or buy a Chiltons or similar small engine repair manual covering your manufacturing year and engine make for complete diagnosis and repair instructions. About $15-20 box store or Amazon, $2-10 used through Amazon.
After problem is solved, I would change the oil because if it smells like gas it is probably significantly thinned and broken down by the gas.
