Oil Pressure Reading Varies With Lights ON / OFF...

Yes, the grounding of the gauge is not perfect (i.e., the wires are not huge, and the connections are not perfect = they have small resistances), and it shares a ground with the dash lights. When you turn "on" the exterior lights, that ground "floats" up to a slight non-zero voltage (due to those small resistances). If the needle motion is about 1 needle width = sort of normal situation. If it's a lot more needle motion than that = might be useful to clean/redo the ground connection to that gauge and/or add another ground wire in parallel to the existing ground wire going to that gauge.

 

Exactly. You may also notice that with the ignition on, motor not running, the oil pressure gauge will move to the right. PITA to get behind there. I just learned to live with it on my C/4.

 

Esben
I put a thick black wire from every negative ground connection to chassis direct to negative battery.

Like this mentioned
https://www.ferrarichat.com/forum/threads/sticky-mondial-8-qv-3-2-t-repair-thread.209843 #3

 

There is a ground post on the lower left side of the speedo cluster. Close to the iso accessory plug. When the dashboard is out that's an easy upgrade. (Unfortunately, any serious work on the electric ancilaries requires to drop the dashboard.)