Okay, the auto mechanic probably turned the mower up on it's side to work on it but had the wrong side down - the carb side. Gas and oil have contaminated the air filter which must be replaced. Also check oil to see if the level is too high or if it smells strongly of gas - sometimes in this case gas gets in the oil. Change oil if needed. If the oil level is not over the max line, it probably is not too diluted and can be used. Any gas in it will cook off anyway. Check the oil again after using the mower the first time to be sure.

Given the time of the year, if you will change the oil and the air filter you may as well go all the way with the winterization of your lawn mower. Remove gas after the last mow, clean the crankcase, sharpen the blade etc.

Here is a link that might be useful: lawn mower winterization

'clean the crankcase'

That 'organic doobie' must be badast shezit, dude.