Small rust colored dots in black iron pot

It should come off with just a sponge and water.

Soak it with vinegar

It's a 5 gallon pot can't fill with vinegar. So
Maybe scuff it of the sides and wipe it down then oil

Use mineral oil if you are going to use oil

Small spots are ok just wipe with a cloth and use it again. When cleaning a pot that big its important to warm the pot this makes the pot dry completely and the metal expand slightly for better oil penetration. If ya putting ya pot up for a few months then food grade mineral oil is definitely the way to go cause it dont go rancid on ya. When ya wash ya pot and just pass a rag in it then oil it this is what happens ya need to heat the pot to completely dry it.