Mines a mechanical gauge with a larger bulb 3/8" NPT into a 1/2" NPT adapter that came with it from autometer, and I used a 1/2" NPT brass T at the oil cooler fitting, the adpaters for the hoses where they go into the cooler ring fitting are 1/2" NPT.
I used a 1/2" close (short) nipple on one end of the T into the oil cooler adapter, the sender into the top of the T and it hangs in the flow just enough to make a reading and not block any flow.
The other end of the T is where the stock oil hose adapter fitting gets screwed into along with the hose.
For a 1/8" sized fitting there's probably better places to put it such as down at the idiot oil light and turbo oil feed line block.
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I like that location. I'm going with that. is that the cooler IN or OUT?