JeffDG
Touchdown! Greaser!
So, if you have a plane, and let another pilot fly it and tell them to just "fill it back up when you're done", and that pilot only has a private certificate (or any certificate and a 3rd Class Medical), that pilot is in violation of 61.113(a) according to the FAA.
http://www.faa.gov/about/office_org...2013/stoner - (2013) legal interpretation.pdf
http://www.faa.gov/about/office_org...2013/stoner - (2013) legal interpretation.pdf
In the hypothetical example you provided, you as the pilot are obtaining an airplane from an individual who is not charging the full range of costs associated with operation of an airplane. Thus, depending on the terms of the arrangement with your friend and the purpose of the flight, you might receive some financial benefit that may fit under the broad interpretation of compensation under §61.113.