They can run all day, all year at full throttle with no adverse effects.. On mechanical hour meters the hours add up faster at higher RPM though, so unless you need the power, choose a higher gear... If you're putzing, 1200-1500 is good, and if you need the power, don't be scared to make it work for it's living... We had a little JD950 (Yanmar built) we put 5500 hours on, it was seriously underpowered for what we were asking of it, spent most of it's life with the "overheat" light on when doing heavy field work, never had to touch a thing on the engine.. Transmission was getting iffy so we sold it.