I would almost bet that you have a big leak with that much oil disappearing. Check for oil on the underside of the car and try to locate all your leaks and fix them before you get into anything too serious.

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1985 300D Turbo ~225k
2000 F350 (Powerstroke) 4X4, SWB, CC, SRW, 6spd ~148k
1999 International 4900, DT466e (250hp/660 ft/lbs), Allison MD3060 ~73k