I have two thoughts for you. If you install a bypass oil filter, you could put the temp gauge in the feed line with a T fitting. If you don't want to do that, the existing gauge is located in the engine valley adjacent to the oil filter housing. You'll have to remove the oil filter housing to get to the location, this involves disconnecting the fuel lines at the the fuel filter, removing all the intake air parts from the filter back to the turbo. The threads on the existing senders aren't NTP (national pipe thread) rather, they are SAE (like those used on nuts and bolts). As I recal, they aren't metric. The parts seal with o-rings. A company called Parker Hannifin has parts that will allow you to put a T in this location.
Search their web site www.parker.com
for "tube fitting". You'll get to an electronic directory, select "hydraulic - tube fitting division", select the link to Tube Fitting Division, goto the pull down for literature - select catalog, click on the link you'll get a pdf. What you want is the Seal-Lok O-ring Face Seal Fittings. From there, you're on your own.