Here is a quote from Bill Kondolay of DTT:
the average temps going downt the highway in the cooler line is between 160 - 170 range, in the pan it is going to be between 130 - 150 so yes it is faiirly closee. However the heat is not generated cruising down the highway in lock up mode,stop and go traffic and in reverse is where the most heat is generated. It is not uncommon to see temps of 205 in the transmission outline . Lets follow that oil, say the oil comes out of there at 210, it is going to the passenger side of the engine block where it enters the water to oil cooler. If your engine is operating at 160 degrees it is going to turn the transmission temps to approx 160 - 170 degrees . From there it goes to the transmission cooler which is located by the front bumper. This is going to drop the transmission oil temps by about 20 degrees . From there it goes back into your transmission through the intermediate shaft. If you have a temp gauge in the pan ,you may see a 5 degree increase over your normal temps driving down the highway, which wouldnt cause you any concern. In reality the transmission oil has been heated to over 200 degrees , and the same senario would be true if your transmissioin hit 300 degrees. The way the oil is routed though the water to oil cooler , through the front of the vehicle where it is then cooled by a oil to air cooler then back through to the transmission via intermediate shaft where your pan temps may register as high as 160 -165 which wouldnt cause you to panic or change your transmission oil.
Again to me the main purpose is to help me determne when to change my oil. A typicial vehicle should service their vehicle every 15,000 miles . I have personally witnessed in my own shop having to change a customers oil after only 6 miles, he took forever trying to back up his trailer and was well over 350 degrees from the line , we had to bring the truck back in and change his oil.
For me, I listened to the expert and purchased a line with the temp bung already in it from Mass Diesel! JMO