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I also have a 2008 6.4 and started having a high oil temp problem during normal highway driving. It just slowly kept going up until it hit 260 degrees then the engine went into a low power mode. This happened from my house to about 18 miles down the road at 70-75 mph.
Based on everything I was reading on the forums, I finally broke down and spent big bucks to have the oil cooler replaced. This kept the EOT at about 242 and seemed to fix the problem for a while, at least it kept the engine from going into low power mode, but about a month later the same problem started up all over again. I have heard of people replacing the oil cooler several times to fix this issue, but this seemed stupid and for sure I was not going to do that.
Here is what finally worked for me, and I wish I had done this before replacing the oil cooler the first time:
I disconnected the radiator hose joint directly behind the radiator at the bottom and let all the coolant drain out. Next I put a water hose in the de-gas container and ran fresh water through. Next I reconnected the hose temporarily and filled the radiator all the way up and disconnected the hose again letting it all drain back out. I repeated this 5-6 times. Next I reconnected the radiator hose and put 2 containers of Prestone Radiator Flush+Cleaner in the de-gas container and filled everything back up to the fill mark with regular tap water and put the de-gas container cap back on. I drove it like this for the next 3 days with a monitor hooked up so I could watch the temperature.
After only 1 day, the EOT suddenly started dropping all at once down to 210 degrees which was within 11 degrees of the engine coolant temp. Obviously there was an obstruction somewhere in the coolant system that was adversely affecting the oil cooler but not the engine coolant temperature. To make sure the engine was cleaned out good, I drove it with this mixture in the radiator for 3 full days and now the EOT stays within 5-6 degrees of the coolant temp even at highway speeds of 75mph. Problem finally solved!!!
At the end of the 3rd day, I drained this mixture out and refilled the radiator with the recommended coolant for this engine and everything has been perfect ever since! I hope this post helps save a lot of people a lot of worries and money.