1998 3800SC into a 1987 Fiero GT- Been looking over the GMTuner wiring harness chart. "Where" is the PIN for the Oil Pressure gauge ?? Did I look and not SEE it ? thanks

i believe it just gets wired from the sensor directly to the fiero harness, id assume C500, id look at the fiero wiring to see where it would go.

Your oil pressure sender tan wire (B) connects to C-203 E. That runs the gage.
Orange/black (A) is conncted to Fuel Pump relay D.
Tan/white (C) is connected to C-203 L

Thank you. that's what I needed to know. WHY, I wonder, is it Not Mentioned in the 3800 wiring Diagram charts ?!

There is info on the Sinister Performance website that shows the C-203 and C-500 wiring as well as an Electrical Interface chart.
The info on the site is invaluable to anyone performing a swap at home, but it is also a really good idea to have a Factory Service Manual available so that any questions that arise while building the harness can be researched and answered. That way there is a good understanding of every circuit in the finished vehicle.
Glad to help you out!