Smell the oil for the presence of gasoline.

This apparently has a Briggs engine.
IF it has a bowl type carburetor, the needle/seat assembly may be leaking, allowing gasoline to eventually end up in the sump while sitting idle.
This severely dilutes the oil, allowing it to easily get past the rings into the combustion chamber.
It would also cause premature wear on the engine.

After a lot of running, you may have evaporated most the gasoline and actually show a low oil level. Let it sit a couple days and see if the level increases.

Doesn't really matter but it's actually a 20023 (R-21SB). Yes Briggs and Stratton. I'll check the oil tomorrow. Thanks.

Just for grins change the oil and use 10 w 30 .If the drain oil smells gassy, Add a fuel shutoff and use it

meatball: Also never tip the mower up with the carburetor (air filter) side down. Oil then runs into the intake tract. Are you mowing on steep slopes? This also causes oil to be sucked into the cylinder.
What Bill told you is very likely to be the problem, but I just wanted to mention a few more possibilities, like an oil-soaked paper air filter. If the mower was over-filled with oil or tipped incorrectly at any time in the past, the air filter may have been contaminated with oil. This will make the mower run rich (excessive fuel in the cylinder) which dilutes oil it contacts and burns it. An extremely-clogged air filter could do the same thing.
So as you can see you have things to check, the first thing being oil level and condition.

Actually the filter was changed last spring, and again immediately before I started it up before this problem(again this has been a lifelong issue for this lawnmower, but to a much lesser extent). Maybe it's normal? The mower has not been recently tilted, but has been used on a slope no differentley than in past times. The filter I took out was not soaked in oil. The dipstick reads the adequate level of oil.I personally put in the correct amount of oil last year. If the needle is leaking, would a new one and/or a new float solve the problem?

Maybe, a shut off is more positive

I assume you smelled the oil and didn't smell raw gas? It's always possible that the engine is worn after 13 years. If you are in a hot climate and are using 10W30, you might consider changing to SAE30HD. I would chose an oil designed for air-cooled service, like the Briggs & Stratton brand.

Yea, oil smells a bit like gas. I use Honda Power Equipment oil. I put straight 30 in the lawnmower I believe and 5W-30 in the snowblower....I hope.

This post was edited by spicymeatball on Wed, Jun 5, 13 at 18:13

Oil might smell a bit like gas after running, since you'll evaporate a lot of it.
It'll smell a lot more like gas after sitting a couple days, if the carb is leaking through.

It was sitting for 5 days when I smelled the oil. It had a slight smell of fuel.

Probably not gas leaking down then. I suggest changing the oil just in case you used the 5W30 in it. I'm going to try some of the Honda oil - I didn't know they had it.