Odd gage behavior 1990 BFN

Don't forget that there's a small voltage drop to the gauges too when the lights are turned on.

Nighty night.

We often ski late into the twilight on the Niagara River, and return under the stars, so not using the running lights is not an option .

I was baffled how a corrosion in power or ground could result in the gage reading increasing, so I did some research on VDO instruments that settled that bafflement. 

The instrument sends supply power "two ways". Through one coil and a resistor to ground, and through another coil towards the "sending unit", which is actually a path to ground, the resistance of which varies as the temperature changes. The two coils both act on the needle in opposite direction, the first one to pull the needle to zero, the other to pull the needle higher.
If the first coil... let's call it the reference coil, does not have a free and easy path to ground, the current through it will reduce, allowing the needle to be pulled higher.
Thinking of the ground buss like the drains in the house:
All the things in service are using that drain. It might be fine when the sink is on, or you are flushing a single toilet, but if the main line has some resistance because of a diaper blocking half of the pipe, when the sink, washer, and a draining tube are all trying to drain through it at once it can back up. 
In electrical terms, the ground voltage rises.
So, when the running lights are turned on, and the ground has resistance, the voltage in that ground leg rises... the current in the reference coils of the temperature gage and the oil pressure gage is reduced, so the balance within those instruments is affected in a way that results in the gages rising. 

It's that simple.  

It's just easier not to use your lights during the day and focus where your going rather than looking at the gauges at night 😀

Some have had good success by using positive and negative bus bars to eliminate the daisy-chain problems.  

That's pretty much normal gauge behavior for anybody with that era Correct Craft, some more than others depending on the condition of your wiring and connections.