Rebuilt 502 EFI's into 540's w/ Merc blue 500EFI Injection and Intake system. Base timing 8 deg. Large (3"x15") aftermarket Oil cooler w/ thermostat. Closed cooled using original 502EFI exchangers and pumps, new impellers. Both engines: I can run all day @4000RPM and the coolant temp will be around 155-160 degs and the Oil 220 degs at the oil filter input port. At WOT the Oil will heat up to 265deg but no higher, and the coolant to just shy of 180deg. When I back off, both will drop slowly. If I come off plane the oil will drop back to 220 in less than 1 min but the coolant temp tends to to stay close to 180 deg unless I run the RPMs up in neutral which will drop the temp immediately. Anyone think I have anything to be concerned about? (Running Amsoil full synthetic 15/40 diesel/marine)

Your numbers sound high compared to my 525efi. Water temp is a steady ~150. My oil temp will go up to 192, then the drops right back down to ~155.

You need to see what is up. If your cooling system (water and water/oil cooling) is totally fine, your tune isn't. Not uncommon for an engine not being tuned correctly having high temps.....your engine + gauges are warning you...as it appears you are listening.

So...cooling system 1st...obviously.

What tempter is the antifreeze good for before freezing? Run as much water as you can with the antifreeze it will cool better. Straight antifreeze will make it run hot.

What is your water pressure?
Steve

I run twin 540 Merlins, my numbers are very similar to yours, seems pretty close to normal , I run 140 Tstats w/ crossover, no circulating water pump on the engine, mine have ran great for 7 summers w/ no problems, my water press. is 20# on both engines, last year I switched to 20 w 50 synthetic racing oil with Zinc in the oil and it cooled down my oil temp a bit to 235 -240 max, if I run hard, which is rarely, cruising for an hour at 60-65 oil stays at 200-205, and I have a big oil cooler.

Your oil temp of 235-240 is on the high side of 'good' range. His 265+ is not.