"G.R. Patterson III" wrote in message
I have proven empirically that keeping the tank switched to L or R helps.
Both
the local ramp by the fuel pumps and my tiedown are on slopes. If I fill
the
tanks to 1.5" below the filler tube (which is normally safe), park on my
tiedown, and leave the selector in the "off" position, the right tank (the
uphill one) will be down about 1/4 tank a week later. Leave the selector
on R,
and I'll still have full tanks. Similarly, if I leave the selector "off"
while
I'm fueling, frequently the downhill tank will overflow before I finish
paying
for the gas. I have developed the habit of leaving the selector on the
uphill
tank, but I cannot give you a good reason for doing that instead of the
downhill
one.

Thank you so much for the empirical results.

The newer maintenance manual (not my original one) references the
desirablity of leaving the fuel selector in "L" or "R" position when parked
to prevent tank to tank transfer. This implies that tank to tank transfer
can occur when the selector is either in the 'off' or 'both' position.

The bare spots are finally growing over in front of my hangar after 3 years
of the 'killing leaks'.