Take a look at this statshot I dug up from The Onion:
It's funny but true. I'm always trying to live as car-lessly as possible but fuel efficiency matters. Emptying your trunk and vehicle of unnecessary clutter, especially seasonal items - or bowling balls anyone?- helps. How much useless stuff do you have piled up in there? Empty that trunk and don’t let your automobile become a storage facility with wheels! For every 50 pounds of junk you carry around, you lose about 1/4 miles per gallon.
As for putting more "oil in the oil hole," changing the oil will also help but be careful: One quart of oil leaked into groundwater can pollute 250,000 gallons of water.
If you change the oil on your own car, never dump the old stuff on the ground or into stormsewers, and don't put it in the trash, even in sealed containers. Contact your local sanitation authority for information about recycling it. If a garage handles your oil changes, ask if they take part in a recycling program. Locally, you can contact the Spokane Regional Solid Waste system for more information which has a list of disposal sites.