S - Sender (should be a single Tan wire)
I - Power/ignition (should be gray wire, goes to (or from) the voltmeter)
G - Ground (should be a black wire, goes to (or from) the voltmeter)

There are the light wires as well (black and purple).

Looking at the gauge, going clockwise:

Top (12 o'clock) is tan (sender)
Right (3 o'clock) is gray
Bottom (6 o'clock) is light (violet and black, violet got angry with me for calling her purple above!)
Left (9 o'clock) is black.

AM General Parts:
5744455 - NA trucks '96-'98, maybe TD too
5745120 - '99 and later TD except Alpha

AC Delco - GM Part:
19244500 - 3-pin

GM Part:
12558074 - For the single-pin sensor

5744455 is the 3-wire sending unit / pressure switch used in '96 through '98 TD trucks. It is correctly identified in the '96 manual, but is incorrectly listed in the '97 and '98 manuals as the unit for the N/A trucks "ONLY". It was used for both the N/A and TD trucks in '97 and '98.
In '96 and earlier N/A trucks (and the gassers), P/N 6000092 was the oil pressure switch, with a separate oil pressure sending unit (P/N 6001037).

In 1999 AMG changed to the 1-wire sensor P/N 5745120. The change was caused by the PCM's takeover of the lift pump operation (based on engine RPM instead of oil pressure). There was no longer a need for an oil pressure switch -- only a sensor for the gauge.

More confusion is also available: The 1-pin and 3-pin sensors use the same connector, install in the same location, and look almost identical but for the pin population. I am not sure, but I believe that a 3-pin sensor can be used in place of the 1-pin sensor, though not the other way around.