[sciguy125]

Alright, according to the law of conservation, your oil can't just disappear. You're probably either burning it or leaking it (there's also the remote possibility that someone is taking it). The thing you should figure out is if it is going away when the engine is running or when it's just sitting there.

If it disappears when the car is just sitting there, you probably have a leak somewhere. As Greg pointed out, you should try putting the cardboard under your car. If you're losing bottles at a time, I'm surprised you don't see it on the ground though. The other thing is you should make sure that your actually filling it up. Put in whatever oil makes the dipstick say full, then run the engine for a few minutes and check it again. It should go down because the oil filled previously empty crevices. If you're losing that much oil, this shouldn't matter, but still... Another thing you can do is clean the engine, then look for dirt after awhile. Dirt likes to stick to oil. If you hose the engine down, you will wash away old dirt and you can start looking for new build ups. You might want to have someone who knows what they're doing do this because water is bad in certain places. With your sparse driving habits, it could take a very long time for the dirt to build up though.

If you're losing oil while the engine is running (particularly at highway speeds), you're probably burning it. In that case, you might want to have a mechanic look at it. However, I had an issue where the leak was only apparent while the engine was running. A gasket toward the top was bad, so it only leaked when the engine was pumping oil near it.