I think 240 after 5 miles is in the ballpark, but i wouldn't restrict the inlet side of the system. I was always told to restrict the water leaving the boat to control water temp in the engine. The pressure, 1-2 Lbs @ 3500 is scary low.......Why so low? A standard automotive cooling system runs above 12 lbs, what you have is a trickle passing thru the oil cooler and engine. Open the gate, and restrict the water leaving the boat with a cable operated ball valve, your oil temp should come down and water temp up slightly.........Just my .02...........Moneypitt