When I added my external B&M cooler I got there special holder that allows you to put a temp sender in it. I just added the gauge to the 5 I already have. This oil cooler has a fan that turns on when the oil temps reach about 210 and shuts off whey they have gone down to I think 165. So far driving around town and racing autocross my tranny temps stay nice and low. I love this cooler not to mention it will save my auto tranny as well.
Just remembered that fitting came from autometer not B&M. It is a special adapter for installing oil temp sending units. Do a search on their website and you will see it.