It's a no-no. HDPE in natural gas gathering service is usually derated by 50% to compensate for small amounts of "gasoline". The temperature design for HDPE is 72 degrees (i think thats it) as a base and temperatures above that call for derating also. The componets of gasoline cause the HPDE to get soft and swell. I've even gone in and did work on 4" pipe that hade about a 5" OD after being in service for 2 years on gas that is very high in gasoline content. This was before the HDPE manufacturers added a derateing for it.