[77newell]

Well, the Perkins manual I have states a 100 hour interval, I agree with the statement above that 50 hours is good. So, I would be comfortable changing no less than once per year and then using an interval of 50-150 depending on length of individual runs, mine are short so lower hours makes sense, if I needed to run 6 straight days I would do that and then change. Given the varied usage conditions there is no "sure thing" answer, you need to make choices you can live with.