The factory GM fuel filter assy for the 6.5 includes a heating element that projects upwards into the very center of the fuel filter element. It is effective in keeping the fuel from gelling, assuming the heater portion of the filter assy works as advertised. I'd imagine it could keep even summer #2 from gelling, but there is no fuel heater in the fuel tank. This means that badly gelled fuel could plug the fuel strainer on the pick-up.