355 oil pressure gauge & sender questions (also in tech)

just to cover the bases, this is also in tech:

Got out the 355 for a drive and noticed 0 oil pressure. The warning light would go out after startup, though, so I suspected a gauge or sender problem. All other gauges, warning lights and instrument lights worked fine.

I checked the sender and it has two leads, one for the gauge and one for the warning light. The warning light side worked correctly, i.e. no light ever if disconnected but light works properly when wire is connected to sender (note to all - be sure you have an oil pressure warning light BEFORE startup or you probably don't have one at all!). It seems to be a variable resistance sender, and I measured under 10 ohms key on, engine off, and 80-150 ohms or so engine running, with variations in rpm. So this seems normal, although I don't know how the gauge is calibrated. The gauge did nothing, regardless of open resistance (sender wire disconnnected from sender) or shorted to ground. I also pulled the gauge cluster and checked the wire coming into the gauges. Oddly, this wire showed from under 10 ohms (engine off) to 40 ohms or so (engine running).

I therefore suspect the gauge (it is a pair of gauges for water temp and oil pressure), but I have NEVER had any electrical gauge failure in any vehicle of any age.

My service manuals don't seem to have a wiring diagram and I don't want to throw expensive Ferrari parts at the car.

Anyone have any thoughts to add or similar failure? I just have this feeling that I might be missing some easy but peculiar electical gremlin that has nothing to do with the gauge. For example, why do I have a different resistance for oil pressure when I measure at the connector for the oil pressure gauge as opposed to directly at the sender?

Thanks.