The first Cub Cadet I bought had a K321 that burned about 1/3 of a quart (or more) with every tank of fuel. You did not add gas without adding oil. The only blue smoke it blew was on start up - usually one big puff and then it ran clean and it had pretty good power.
All it needed was an overhaul, the top of the piston was worn a surprising amount and I was told by a knowledgeable person that it was caused by dirt from either running without an air filter or a leaking filter housing. The rings were also in poor shape but none were broken.
It will run a long time burning oil but all it takes is forgetting to add it one time and you'll have some serious issues.