I use the Aerotrend "Easy Flex" gas/diesel tubing. It's light blue transparent. Dick Bennett sells it. Tower used to sell it too, but apparently not any more. It is advertised as non age hardening, and so far it has worked well for me, but I think it will still harden in the long run.

When I get around to order from Tower, I might give this a try for in tank use:

http://www2.towerhobbies.com/cgi-bin...?&I=LXRZ80&P=6