A lot of us go overboard here with maintenance, specifically oil changes. While their worth is difficult to prove, I do believe that cleaning the air filter on OPE is something really worth going overboard with. If it is really clogged with dirt and grass, I'd consider doing it almost monthly during the mowing season.
Dirty/dusty air getting past the filter can lead to oil consumption as cylinder liners become heavily scratched by hard particulates.
Check the manual to see whether it is supposed to be oiled or not.
PhilsSmallEngine, I don't doubt the quality of Amsoil's 15W-40. However the same could be said of any 15W-40 in comparison to a 10W-30 ... and many are available locally at a lower cost.