VDO Voltage wiring help

I bought a set of VDOs, ive wired them up to light up correctly so thats a start.

Ok so ive had a look at a couple of threads but still have a couple of questions.

I understand the  Voltage needs to be fed from an ignition fed supply. The gauge has 3 terminals on the rear (excluding lighting) a positive and negative and one other. Id have thought id only need a pos/neg, tried and didnt work so whats the other one for?

Oil pressure, seems people advise busing connection on head rather than filter, ok, where can i get the t piece…

I may have more question as i go, appreciate any clues…

Anyone?? Someone must have some info..

The connection on the head has a lot lower pressure the on the filter. I'd have thought filter was the best point (this is where I put mine), although I'm no mechanic.

I got a t-piece from an aeroplane place, it had the 1/8 tapping on the side I needed.