The wall thickness of the HDPE pipe is greater than for the steel pipe of the same diameter. So you have a smaller inside diameter (id) for your HDPE which increases the friction factor and therefore, higher pressure loss. (The friction factor for the same id pipes is lower for HDPE pipe). Depending on how heavy the oil is, the flow may be non Newtonian and therefore the hydraulics may be different for each pipe due the id. Hope this helps.