No. The oil filter can will give you a decent reading, not the block.

How close of a reading are we talking?

Pretty close. The factory oil temp sensor is mounted to the remote oil filter housing on my Fountain. I read abt 10 degs cooler with the IR gun compared to the oil temp indicator, but who knows how accurate the 24 year old factory gauge is.

tell us more about the engine..also are you using stock merc cooler lines etc ? stuff is pretty much junk from the lower HP engines, best to re-do with larger lines, better non restrictive fittings, cooler, adapters...

+1 on everything SB said. I had a ZZ502 with 300* oil temps after a WOT romp. Changed to bigger lines and bigger cooler. And to lower the engine/block temp I drilled three 3/16 in. holes in the 140* thermostat. Lowering the block temp helped allot!! Used the same set up on my NA 557 with good results. Oil was always Mobil 1 V-Twin 20/50

Sounds like the oil pressure relief valve (located in the block oil filter pad) is bypassing the cooler. Many plug this valve so all oil is run through the cooler

While it could be what you say Kurt, however remote, There may be another possibility here. Here is a lesson I learned a few years back. I had been using 3x18" coolers for years in my old engines and always ran between 210-260 degrees maybe even 270 after a long hard run. When I built the new engines for the phantom I bought the same 3x18 coolers. But the oil ran so cold ( 100-120 ) the oil condensated. I really had no experience at the time with oil thermostats because I never needed them . The reason for the difference , my original coolers, which I got through a diesel supply company, were very different. They had a bundle of 12 tubes while my new ones bought from a offshore company has a bundle of 32-35 in them. Just a thought.

I'm guessing that the actual tube length for your oil cooler is somewhere around 12". Still, 12" x 3" diameter is a fair amount of cooler unless you have a high H.P. big block (over 425 horsepower).
#10 lines should also be plenty. It is easy to sight through the tubes to see if they are blocked (impeller pieces sometimes).

I would not want to see 300º oil temps under any condition, even WOT for brief periods. I would ensure you gauge is reading correctly and then
follow the many great suggestions given. I too use Mobile 1 V-Twin 20-50 oil.

My first thought was a lean condition as well, could also be timing. So those items need scrutiny asap. Also along the same lines as kidnova mentions, I'd remove the coolant stat and try to get coolant temps as low as possible until you get a handle on the issue. Oil temp will track along with coolant temp so any improvement in coolant temp should translate to the oil as well. Plus of course high quality synthentic oil is a must until you get the temps under control. Anything over about 270 calls for throttle back and crack hatch.

If your engine is computer controlled it may not like the switch from a 160º to a 140º thermostat...just something to consider.

Captain Yarrr, with your pumped up 6.2s I would recommend oil temp gauges. You might be surprised at how hot your oil is getting.