About 7 years ago, I purchased a used 1976 Bolens G152 (diesel) Iskea that was well used but also ran well. The oil would turn black as soon as I changed the oil and filter, and I change it once each fall at the end of season...usually about 70 hours/year. Never got better after each change. Two years ago I purchased a new TYM 233. I follow the same practice, and so far it is staying very clean. Don't know what all that means, but I'd stick to regular changes of oil and filter.