It all depends on how many cartridges you fire in 12 months, but I would suggest that once a year is quite enough for most guns – providing you have a thorough normal cleaning routine after every use and you are particularly careful if the gun gets very wet or dusty.
As far as oil is concerned, pretty well anything sold as “gun oil” in your local gun shop should do the trick.
Don’t use motor oil; it is far too thick.
Guns need only a tiny amount of oil inside the action.
Too much oil can gum up the inertia mechanism which transfers the trigger to the second barrel, and also soak into the wood and soften it.