Season's Greeting to you.

My '99 Durango shows P0505 code, the Fuel Gauge dropped to empty, and anotehr error code concerning the Governor Sylenoid is showing.

All three symptoms started at the same time and it would be a looong shot for all three symptoms to just start at the same time.
Reset the computer in all different ways known and without starting the car, the P0505 pops up, fuel gauge will show full and then moment later will fall to zero, if car started and shifted into drive right away the car will only have first gear until you run it up to about 4k rpm at which time it will shift.
If I start the car and let it run for a little and then shift it will start driving in secoind gear and then shift just fine, just not into first.

Batter charges fine, holds charge fine. Alternator charges fine, works fine.
Throttle position sensor had thrown a code and was replaced as well.

No missfires at all.

This all started together about 20 minutes after an oil change to all synthetic oil. Person doing oil change spilled a little oil but not much.

All fuses, relays have been checked, switched,played with...no change.

Is there a place on the car where all three wires to these components meet and maybe short out?

 

Both of those codes are related to the idle control valve. Id replace it and work from there

Faulty Idle Control System motor
- Idle Control System motor harness is open or shorted
- Idle Control System motor circuit poor electrical connection
- Faulty Powertrain or Engine Control Module
- Vaccum leaks or damaged hoses or improper routing

 

if you think something shorted by being spilled on, look in the area where the spill occured for any vulnerable connections