You may change your oil every 3000mi, but you should do it every 3-4 months regardless. Oil can be contaminated even though it is sitting. Moisture contaminates the oil when it sits.
Anyways, to answer your question a few things can cause oil leaks. If your PCV system is plugged up it can cause excess pressure in the crankcase and blow out a seal or gasket, so make sure your pcv valve is operating and the hose is free flowing. Contaminated oil can also cause oil leaks, althought it sounds like you change your oil quite often, so I wouldn't be too worried about that.
You may also be burning the oil through the engine. Your vehicle is quite old, you may have a bottom end problem or leaking valve seals.
Sunday, February 1st, 2009 AT 3:29 PM