I have a Murray 22inch. A few weeks ago my Teenager said it started smoking. I added oil and it looked OK. Actually, she added 2-cycle oil; I saw it, dumped it out and added SAE 30; a bit got into what I think is the air filter and it smoked a little from that (but no longer from lack of oil) and then it ran OK. For a few weeks.
The other day the Nephew was mowing the lawn, and said it started smoking. There was oil, but maybe a little low, so I filled it to the line. Now it smokes really bad out of the alleged air filter. I'm not aware of anything getting into the device. I intend to take the filter thing apart and hunt down a replacement element for it (if that's what it is), but I saw some mower repair posts in this newsgroup and thought I'd ask.