I know this thread is fairly old, but I'll chime in for the sake of posterity
. The Greasecar WVO conversion kits come standard with pex hose-in-hose to supply fuel sheathed in coolant-filled heater hose. I've had this system mounted on my Mercedes 300D for the past 3 years and probably at least 30K-40K miles, and have not had any problems. The heat exchanger in the tank is copper, but this gets less heat than the pex. I'm sure the copper is a better heat exchanger, but if you don't need that much heat in a particular part of your system, the pex holds up just fine. Even the brass compression fittings that hold it in place are still holding fine.