NotTaR of small Gasoline Engines and Rotary Lawn Mowers : Oil change                              
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Oil change

If you don't do anything else to prolong the life and happiness of your small engines (and your automobile, for that matter) it should be an oil change at the recommended interval. Oil loses its lubrication and cooling effectiveness with use and this will gradually take its toll on your precision engine parts. Even a simple lawn mower engine is machined to very precise tolerances and any contamination will increase wear. Ultimately, its performance - starting and running - will suffer and its life will be shortened.

Since there is typically no oil filter, all the grit, metal particles, and other undesirable stuff continues to circulate with the oil to find its way in between precision engine parts.

For small engines, the oil change interval is usually specified to be about 25 hours of use. More frequent oil changes may be desirable if the equipment is operated in an extremely dusty environment.

A typical mowing season for a modest size lot is around this amount of time so an oil change once a season is probably satisfactory. I recommend this be done at the end of the season so that the old contaminated oil does not sit in the crankcase during the winter months and you will not (conveniently) forget to do this at the beginning of next season when you are eager to get at that straggly lawn.