There are 3 rings on the pistons, you can have good compression rings and bad oil control rings and it can use or burn oil.
Pull off the valve cover and run it, if it's smoking out of the top of head then it's valve guides and seals, if it's smoking from the block out the vent holes in the head then it's rings.
Smoke out the exhaust can be a injector problem it can also be an injector pump problem. I would bet money on a leaking injector.