A good borescope, spark plug and ex pipe inspection along with leakdown and Allenís suggested compression test may indicate only 1 or 2 cylinders need to be pulled. Glazed cylinders is seen as brown patches on the cyl walls.
If its only 3.5 hrs/qt. and all other tests are negative it will be hard to know whether to run it or do surgery. Certainly increasing oil usage is indicator to pay attention to.