I have the center console, i removed the clock. I put a oil pressure gauge in the hole, ran the wire to the dual sender. I want to jump the power and ground from one of the other 2 gauges. I figure between oil temp or volt ; go with the volt gauge. It is already switched, but then i was thinking how this voltmeter is affected by the brake lights, turn signals etc.

Is this a bad idea?

Why run more wires to the fuse box if you dont have to??

The clock had a + and a - wire for it, right?

Joe! You have to spring for the new wifi gauges!

Don't use the power from the clock. That's always on and will drain your battery without any need.
The power from the voltmeter or oil temp is exactly the same wire and is good to use. Don't worry about fluctuations caused by other loads - everything in your car is designed to overcome those fluctuations.
On a separate note, big voltage jumps when something is turned on point to a bad ground on the circuit you just turned on.

The volt meter is what's called in the trade "eye candy". It's readings are essentially inaccurate and irrelevant. If you want it to give you a true picture of the voltage at the battery instead of the fuse box, you need to use the power wire from the fuse box to activate a relay into which you run a lead from the positive battery post. If you don't know how to wire a simple load reduction relay, just ask.

The Cap'n