Mower has been working flawless for 3yrs. Just today on my lawn side hill it started smoking bad. { blue smoke}. On the level ;.....fine, ??? WHAT THE HECK
Early signs of a blown head gasket.The Intek single cylinder engines have had ongoing issues with head gaskets blowing between the cylinder wall and the push rod cavity.
Blue smoke is oil, and you can bet on that. Gasket leak suggestion is sound advice. Compression test can be helpfull, but if leak at base of cyl, may not show up as low compression. Some Briggs engines don't do well on slopes, but since your trouble only recently showed up, gasket is good bet.
Left side down is the side with the push rod cavity and the area where it blows the head gasket. Oil pools in that cavity where it gets pulled in with the intake stroke.