I tried doing this myself on my Porto 20vt, I didn't find it was that straightforward, never having done it before. The procedure is to unscrew the low oil pressure warning sender and screw in the gauge. Start the engine and let it warm up until the fans come on, and see what the dash gauge shows at hot idle conditions. Then switch off the engine and press the "release" button on the external gauge to drop the pressure reading to zero (some oil will likely leak out at this point) and restart the engine and leave it at idle, noting the readings on both gauges. Then increase the revs in steps, say to 1000, 1500, 2000, 2500, etc. ...easier said than done if you are holding the external gauge and a notepad and pen whilst looking at both gauges and trying to keep your foot steady on the pedal...
On the 20v/20vt engine the spec is pressure when hot (oil T 100°C) at idle should be > 1 bar and > 4 bar at 4000 rpm (but dash gauge stops at 3 of course). On the 16v/16vt engines the spec is 0.8/1.1 bar at hot idle and 5/5.5 bar at 4000rpm.
How long has the oil been in there, when was it last changed (what date, and how many miles ago)? What's the engine mileage?
Low oil pressure can be caused by worn bearings (eg: big end), worn oil pump lobes, problems with the pressure relief system, and dilution with petrol from leaking bores/rings/injectors - try a "sniff test" on the oil, does it smell like oil or does it has a faint whiff of petrol?
1996 Portofino 20vt & 2000 Pearl White Plus
1985½ & 2016 2017 Fiat 124 Spider + XF Sportbrake