Oil can only leak two places, internal or external. External you can find easy enough especially at that rate but internal can be a few places.
Through the valve guides because of bad seals or worn guides, rings either sticking or worn and the PCV system. Valve guides and seals are easy enough to spot, you will get a puff of blue smoke when the engine is cold started, the cat has not lit off yet and you will clearly see the blue smoke.
PCV is easy too, just pull the hose, if its dry its not going there if oil is there you are getting blow by or a plugged breather or PCV/tube. Rings are another story, the compression and leak down test can be fine but the oil control rings could still be sticking as they have much less tension and thinner than compression rings. Worn rings can be found with a leak down test or even a vacuum gauge. Going to HM or thicker oil is not the answer for sticking rings, it only masks the issue.
With less than 100K I doubt the rings are worn out unless the engine had poor maintenance with the cheapest oil. First thing after checking the things you can see is to put a vacuum gauge on the engine, if it has 18-21 Hg then the rings are probably not worn to the point they would cause an issue.
A good ring cleaning might be in order. Its very easy to perform on a inline engine and usually has good results.