You can take used motor oil to either of the Fairfax County waste transfer stations (Fairfax or Lorton). There is a huge tank to pour it in.
http://www.fairfaxcounty.gov/dpwes/trash/dispsites.htm
Look for where it says 'motor oil' on this facility layout map for I-66/Fairfax:
http://www.fairfaxcounty.gov/dpwes/images/recycling/i-66-map.jpg
Same for Lorton:
http://www.fairfaxcounty.gov/dpwes/images/recycling/i-95-map.jpg
Most gas statins that are full service (with a garage and a mechanic) will also take your used motor oil.