I have been using 25/40 merc oil in my new 6.2s. motors run at about 175 degrees 40-50 psi after running awile say 4000rpm to 4500 then then bring down to a idle my oil psi drops a lot 20 as low 10 psi. I keep the engines up a little above idle so the oil psi is up say 20 but if broght to a idle its to low for my liking. why does the oil or engines drop so much in pressure ?? can someone give me any feed back thanks !!

Oil pressure is not as much of a concern as you think. You should be more concerned with oil volume. Think of a garden hose running without your thumb over the end of it. Water flows with a high volume out of it. If you were to put your thumb over the end of it, it creates more pressure but less volume. That is where synthetics come into play, they flow better at high pressure therefore creating better volume also.

Ok, but it still does not explain why the oil pressure drops like he described. Is this normal after running for a period of time at higher rpm's??

When the oil gets hot it thins out , less pressure. Once the oil cools down the pressure will go up.

Our hp500 do the same thing but they do have 700hr on them, And yes they are ready to be gone through this winter.

Running for a while at 4000-4500 will cause the pressure to drop. Hotter oil=thinner oil. If it was too low, the alarm would sound. 10psi for every 1000 rpms is fine. If you want, try running some Mobil 1 15-50 wt synthetic.

Is this common for mx 6.2s do this with the psi, but what about the oil thickness is 25/40 merc oil enough protection.

This is normal for every engine to loss some pressure when it is hot .

is it normal to be at 10psi for new engines then as the oil cools it returns to 40 psi. I was thinking about calling merc to find out.

You need to find out how hot your oil is. You may have insuffecient cooling or a partial clog. Synthetics will hold up to the higher heat and keep the pressure and viscosity (resistance to flow) up.

If you have an infared thermometer hit the oil filter or any where before the cooler and check the temp.

If you do not have one than get some of these stick on thermometers. I use these all the time at work and are accurate and inexpensive. (cheap gauge) stick on your oil filter.

http://www.mv.com/ipusers/paperthermometer/ Industrial Application

The TL8-170 is what you would need. I also recommend these to anyone who does not have an oil temperature gauge.