Nice looking truck.
I'll let someone else verify the air filter oil, but I believe anything SAE 30 or heavier would work.
The filter canister looks the same as mine, a Deluxe Filter, which uses the sock type filter. Is there any information stamped into the top cover ? If it is a Deluxe Filter, it would specify a JC filter element. I recommend the Baldwin JC405. It's a better fit than the Wix, or Napa, filter.
The fuel gauge may be disconnected after the 12v conversion. It would need a voltage reducing resistor to lower the supply voltage. Also, if the sender, and tank, aren't well grounded the gauge won't work either. That is fairly common on these older trucks, especially if the tank has been painted, or coated.
The horn doesn't look right for a Dodge. The back isn't as rounded on the original horns. However, why would you want to put a 6 volt horn on if converted to 12 volts?