My oil gauge reads 300 at WOT. So I went and bought a new oil cooler from Teague marine and saw no difference. I bought a infrared temp gun and took a reading from oil pan and oil filter and even the lines and it never took a reading over 240.

My question is if it was really 300 degrees would I be able to hold my hand on the oil filter? In which I could.

I went though this too a while back Brought my sender and gauge in the house with a battery and did the boiling water trick to verify. Mine were pretty close.

If you can hold your hand on the filter you dont have oil going through it. Check your lines to see if they are reversed or something. If yours is plumbed like mine and you dont have oil going to the filter, you dont have oil going to the cooler either.

Something is amiss.

Where's your oil temp sender mounted?

Are the sender and gauge a matched set? Meaning same manufacturer?

My oil temp sender is mounted in the oil pan. I know I have oil going to the filter because the oil filter is hot but not to hot not to be able to touch it. Plus I can fill the pressure in the oil lines that go to the oil cooler. The sender came with the gauge. Also my water temp never goes over 150. I was wondering if the oil pressure bypass has anything to do with it. Does the bypass cut of the oil filter at high pressure?

the pan will always read hotter. i think

Ok cool, I though you could hold your hand on it. In that case its only 120 or so and not 240. As far as the bypass, I plugged the bypass on my mark 4's. I then run twin filters with their own bypasses so the cooler sees everything. I think the filters probably pass all the oil when its hot but not when its cold. Just a guess.

I have my bypasses plugged and no bypass on the filter mount. The oil pressure sender is mounted on the front of the block so it is reading after the oil goes through the filter. Sometimes on a cool morning I get 100lbs of pressure at startup. I don't think the filter is causing any real restriction even with cold oil. I do think it's possible that the bypass in your block is wrong if you have a gen 5 or 6. Read up on some of the threads here. The oil pan is where you should measure the temp. 300 is too much and yes it can happen with 140 degree water temps. Even with synthetic oil you're living dangerously. Try different senders and gauges until you're sure of the temp.

So, are you saying it is possible the bypass could cause the engine temp to be high? Is it possible to plug my bypass with engine still in the boat?
I have plenty of room to get to the filter pad. If so, what is involved? The engine is a 1992 502 V Mag. Mercruiser

This applies to gen 5 and 6 only. The bypass is in the filter pad on the block. If it is the bypass with the soft spring, it will open during normal operation allowing the oil to bypass the cooler and oil filter. It can be replaced with a stiffer spring bypass that will ensure proper operation of your filter and cooler. I just plugged mine with a small freeze plug. The bypass can be changed in the boat, the freeze plug requires removing the engine. Do a search on "oil bypass", you'll find a lot of info.