i got a overheating problem, my 6.2 get warm when the rpm's go over 2800-3500. but when you average it 2800rpm's it fine. i want to know if it must have a oil cooler and how big it must be?

I believe you should already have an oil cooler.
You'll notice two lines coming from the lower driver side of the engine into the radiator.

2800-3500rpm is really high for normal driving. I would expect the motor to get very hot at those speeds.

Mine usually runs about 1/4 temp after warmed up, 1/2 temp after pulling uphill with a load for quite a while, and anything close to 3/4 I pull over and let it cool down with the engine running.

Generally you want to target 1800-2000rpm for daily driving.
http://www.skylab.org/~zetan/van/GM62-NA-v-turbo.png


-z