Oil Thermostat 575 540 Tall Deck??
 

I recently bough a 1989 Formula 357 with stock 540 Tall Deck 575hp. Does anyone know if these motors have oil thermostats. I see the oil coolers but don't see any additional housing for a stat. I only ask because my port motor oil pressure drops after running for a couple of minutes on plain. It starts out the same as the starboard motor but slowly drops about 10 psi less. When I slow down it drops below 20 at idle while the starboard motor stays near 40. After I've idled for a bit it comes back up and mirrors the starboard. Boat still has factory gauges. My next step is to check the temps off the cooler and oil filter with a IR temp gun but my boating is coming to an end. I have fresh oil in so no chance oil is diluted. If I can squeeze in a few more run I'll try swapping sending units or buy a new one. Thanks

Oil thermostats are there to make sure the oil gets up to temp.
I don't see anything in the Merc diagram about an oil Tstat for youe engine Genuine Mercury Marine parts, large inventory, fast shipping

I get why they are there. I'm just trying to find out if my boat has them or not and if so where they are.

You didn't mention water temps between the engines. Hotter temps thins the oil shows less pressure. This could just be a plugged oil cooler. Remove the hose to the cooler and verify no obstructions. If you have thermostats they might be mounted to the block at the oil filter location.

The water temps run normal, nothing out of the ordinary, around 160 on each motor. I figured if the oil cooler was obstructed then yes the motor would run hot as well. Is there a bypass for the oil line itself if it's plugged up through the cooler? Hopefully I can get it out this Saturday and run it to see if I can get this somewhat figured out over the winter.