I have a motor that is said to use 1 litre a month. kms on that motor are about 400,000km. My other motor I have had since 92, a friend and self put in an oil additive and after that it started burning oil. I also hydrolocked the motor doing a water crossing, shut if off and pulled the glow plugs and used the starter to pump out the water, glow plugs back in and drove the rest of the day and for many years but it did not have the same power. it has about 440,000km on it.How many km?
Did you perform some test?
Was it a sudden change in engine performance, one day fine and the day after an oil burner kind of thing? Blue smoke on start up and running (even when warm up)
Like gimme a 60 says, it maybe something like a valve seal?
As for the engine, hard to say until it is not disassemble.
If oil pressure is good and engine have less then let say 350k km. Chance that crankshaft/ codrod & camshaft bearing are still good.
May need deglazing the liner and new ring. A complete overhaul is expensive and may take all winter for parts to get in or shop to do the work (that’s my storie!)