I can not tell you what to do but I've changed oil in my cars for 20-some years, and now in my equipment. What I mean is, I know what oil looks like after 3 thousand miles in a car, and if my mower's oil is not at least that dirty, then I don't change it. Today I also use at least a synthetic blend and in a few years will be switching to full synthetic, none of my stuff runs on standard oil and this makes a difference as well. They say if you've been running some type of synth that you can drain the oil from a motor and continue to run it WOT for several hours without damage, but it is not recommended. Either way, I know full synthetic when it first gets to the point where it needs changing, is still at least as good as completely fresh standard motor oil. I have two mowers and I run them interchangeably, both machines get hours put on them and I run solo so I cut between 500 - 800 yards / year. That having been said, I change oil and filter on my mowers once / year, in the spring. Should the oil ever get so dirty that it needs changing in-season, I would do that but so far this has not been the case, it hardly turns color.