Generally, if the oil level is going down and there are no drips or runs on the engine or the ground then it's being burned during combustion due to bad valve guides or bad piston rings. Usually when you're driving, you can't see the exhaust emmitted from your tail pipe so it is possible to have oil in the combustion. If you have mechanical skills, go to Harbor Freight and for $30.00 you can buy a "Leak Down Tester". You'll need a compressor also. Once the cylinder is pumped up, if you hear air leaking from the intake side then you have bad valve guides, if you hear air leaking from the exhaust then it usually means bad valves and, if you hear air leaking through the oil fill or any other breather device then you have bad piston rings
Not so general but possible (only seen this once) is a bad crankshaft seal coupled with a super tight mating of the transaxle and the engine. Oil collects in the bellhousing and eventually soaks the clutch. Again, I've only seen this instance once and it's VERY, VERY unusual to have such a tight fit between the engine and transaxle that oil can't leak to the ground.