Buddy of mine is a horticulturist/landscaper, he does excellent work, but his work truck(just used by his helper daily and nothing else) is a mess. Its a single cab, single rear axle F350, last year of the square body style, with the good ol 351. Its used to haul trees, branches, and has one of those heavy Rawson-Koenig utility beds, so you can say its always under a load. It currently has 151k. He got it in 2004 with 70k, and had been running 20W50 Castrol until last year, when i convinced him 20W50 wasnt needed and that he should use up some of the 15W40 Rotella he had sitting up in his barn. been changing it for him with the rotella and a MCFL820S, and at 3k in to the oil change(usually go 4k) its a quart low. I think its the rear main seal. Anyways hes not puttng any more money into it. it runs great, and we just put a new radiator,(it was driven 40 miles with a bone dry radiator, and was fine) so it should last a few more years. I was thinking 10W30 Rotella T in the winter and 15W40 in the summer, topping off as needed. good plan?