if it surges you are getting air, or the governor is sticking, check the small piece of rubber hose near full pump for a hole, its on the inlet side of the pump, and the original filter head has a factory installed return line, with a orifice, and check valve to remove air and return it to the tank, install a 10 micron filter pre lift pump to protect the pump from debris from the tank after all the truck is pretty old. I have run 60% virgin motor oil and diesel with out any problems in factory fuel system.