If it is using oil, it probably needs new piston rings. Usually you can test the rings by checking the compression, then putting a spoonful of oil into the cylinder through the spark plug hole, and repeating the test. If there is a substantial rise in compression, the rings are leaking.
Since you have the crankcase open, it seems a good time to replace the piston rings. Taking the engine out of the mower, and opening the crankcase, are the biggest parts of the job. At that point, replacing the rings is easy, as long as you are careful to do it the right way.
It is up to you of course - we are here to help if you decide to do it.
