I’ve only worked on Cat CTL’s, but the fleet of six259/289/299’s I’ve looked after has totalled some 30,000 hrs. The Advise about changing the final oil at the same time as the engine oil is spot on, and every oil change I’ve performed the oil has been filthy. However, two finals out of twelve have failed the seal between the sprocket carrier and the rear housing of the final, and one totally detonated the gear train at about 3,000 hrs with no warning, despite the wear pattern on the remaining teeth showing nothing abnormal and the final still having the correct oil level. As someone remarked earlier, these machines aren’t designed to be rebuilt like you would a D11.