While it's true that the transmission cooler is on the cooler side of the radiator, the XJ radiator is small enough without introducing more thermal load. That, and what if the brazing fails? The "cooler" for the transmission fluid is essentially a hollow tube (think two layers of pipe brazed together at the ends) and that's not the most efficient design.
Also, the upper fitting is a flare, the lower is usually a QD.
Considering the XJ has a tendency to run hot (I think nominal is up around 200*,) I've never been fond of running hot oil through hot water, and the internal cooler is a cheap solution to a potentially expensive problem (and the excuse "that's how everyone does it" just doesn't work with me.) The thermal valve with the external cooler is a better idea.
For the argument of the water system heating the fluid to operating temp - true, but it takes the engine a while to heat up due to the greater volume of coolant involved, so how much help is it? On some other vehicles I've had where I did the thermal valve, spin-on filter, and external cooler setup - and compared times for temperature shift - the thermal valve won out AND the engine stayed cooler the whole time. I don't have the notes, but the timing was farily significant.
So, all else being equal, I am willing to go on record saying that an external cooler with a thermal control valve is a better option than the OEM setup, and further state that modifying the system to delete the OEM internal cooler in favour of a PROPERLY CONFIGURED (it's not that difficult) external cooler setup is a better way to go - especially for a vehicle that has the potential of seeing little "ram air" cooling (think off-roading,) which will reduce the cooling efficiency of the radiator and associated systems. Even with the fans going full tilt, it's still not going to get as much air as it would at freeway speed - that's just the nature of the beast. If you are planning on off-roading extensively, I'd even go so far as to make you start thinking about an engine oil cooler, and possible a P/S cooler. Bringing the fluid temperature down on P/S (like any hydraulic system) will make it a little more consistent, and cooling the engine oil will help bring overall engine temperatures down by assisting the task of the radiator with a fluid that gets closer to the combustion process, and is directly impacted by internal friction...
5-90
. I also have had great results with my deeper pan that has proved itself really well to remove the slow shifts I was getting in hot sand while towing.
