jjellfern, that's pretty much the way I was heading, I had a digital voltage readout module, and have purchased a thermocouple, I took the direction of the Scangauge II because of the other benefits, plus the fact I didn't have to do any other modifications. In all this, it is not the exact temperature that is important, but the normal, above normal and damaging temperatures that are important, even it the dial is marked green, amber and red, that is all you really have to know.
I was going to glue the thermocouple to the surface of the gearbox oil pan.
My Scangauge should arrive within the next week.

Yes you really only want to know when it's above normal . I'm fitting an exhaust temp sensor for the same reason . In my way past we would just use hose clamps to hold sensor to oil filter to read some sort of oil temp in our rally engines . I like having another system to check as a fail safe way .

OK I have my Scan Gauge, it can show four parameters, what would be useful to have displayed? ATM temperature, Coolant Temperature? and where do I get the codes (I have ATM temp thanks outlaw 40)

Currently running Scanguage 11 in 2015 BT50 Dual Cab GT auto. I monitor Voltage, instantaneous litres/100klm, coolant temp and trans fluid temp. Coolant varies 89 to 96 then up to 100-102 climbing hills. Auto fluid 97 not towing then about 100-102 towing 110 on one hill in Tasmania. Caravan is 2750kg.

I have set mine to KPH, as the vehicle speedo is under-speed, the scan gauge is pretty bang on with the GPS, Coolant temp. , Auto gearbox temperature went for a run today up some hills to run in the motor , went up to 96, ute not loaded, and voltage.

Gave the ute a bit of work to do, and the auto gearbox temperature maxed at 102'. The scan gauge did what I wanted, maybe a bit hard to see if the sun comes out, but they warn you of this in the instructions.