Re: Gravity or siphon gas line
^^ What map said...
Having too much vibration insulation can be worse than none at all... What happens here is you stil got the vibration from the road to deal with, but with too much or too thick of insulation it viges the frame and the tank a "running start" before they contact or pull on the bracket etc, and this accelerates wear exponentailly to say the least.
There are several methods to get it "just right" and your tank will last as long as your bike does... I use an old grip cut in half and glued to the top of the frame then I just bolt the tank onto that and Snug it down, just enough to where the tank won't move and that's it... A drop of locktite keeps the nuts from backing off and it hasn't failed me yet... I do most my riding on pavement but there are the occasional potholes and rough driveways to deal with.
Other people have successfully used a piece of cut up inner tube, double sided tape, a few coats of that Flex Seal stuff on the top tube, undercoating, old gel grips cut up, etc. The idea is to insulate it from vibration and hard bumps, but it needs to be thin.. like 1/8" or thinner is all you need.