paccar mx-13 oil usage?

I have just under 800000 on my 2013 mx13 455hp.truck has never had a motor problem.ive had dozer nozzle
For def filter replaced twice.my truck never needs oil added between changes.have to add coolant twice in four years.motor is under powered but fuel millage is low 7s mpg.i was worried about buying my truck with this motor but very happy i did.btw i am second owner- truck had 245000 on it when i purchase it.