Nobody has really answered the question of where, when, and how often. And there is no definitive answer. Part will depend on fuel used. Part will depend on how often you fly. Part will depend on where you store the engine in between. Most of our own spare engines are oiled and sealed in a plastic bag and stored in a dry area (like a closet, not a damp basement).
Most engine manufacturers use a good assembly oil that will keep them in good condition for a number of years if never run and stored properly. But once an engine is run, moisture will be left inside plus oxides and other residues that can be quite corrosive. And then there is the gumming action of castor residue. Here is where after run oil comes in.
Now, if you use a castor/synthetic blend (50-50), you will have minimal problems with castor residue and varnish and gumming up. But any synthetic leaves its own problems, In addition to the fact that the synthetic will usually burn when in a hot run, thus providing less lubrication when you need it most, most synthetic oils leave those nasty residues that can promote corrosion. If you are using an all or nearly all synthetic oil, your problems in this area will increase.
But alcohol absorbs water, so any fuel residue of any kind remaining inside the engine can absorb water. The nitro and alcohol evaporates off quickest, and the water remains to do its thing called rust.
The key areas where rust and corrosion cause their biggest damage are the shaft and the bearings. The iron piston engines (most older engines, but even some newer Fox engines) have a second problem--the piston. On these you will need to get the piston, itself, lubed for long term storage. (Have you ever seen an engine that was stored on compression where all of the piston was bright and shiny except the portion that was open to the exhaust? Rust!)
Now, in addition to rust and corrosion, gumming can be a problem with ball bearings, piston pins, and piston rings. So:
The key areas of concern center around the lower end. This is fortunate because it makes it easier to add a few drops of after run oil in the intake at the end of the day, give it a couple of flips, and you will probably be OK for several weeks if you don't store the engine in a damp basement. I don't like to pull the plug unless you have to. It is inconvenient on most engines to pull the muffler or pipe. So the intake is where you are going to be oiling the engine most often.
Normally, if the engine is going to be run again in a short period of time, gumming won't be a problem if you follow this regiment of a couple of drops in the intake and work it through. If you are going to be storing the engine for a longer period of time, add more oil. Too much oil won't hurt. Too little and, well, you know the rest. Again, that is one of the strengths of the Marvel Air Tool Oil. It won't hurt the restart as some of the other oils sometimes used for this purpose will.
This is an area where we all (me included) often fall down. We fly one day and figure we will be back tomorrow. And, for whatever reason, tomorrow never comes. And then it drags on for a week, a month, all winter.
When in doubt, put a few drops in the intake and flip it over a few times. Then add a couple more drops. Even then, that quart of Marvel Air Tool oil will probably last most flyers a life time. But buying a second quart is still cheaper than buying a second engine.
(And, no, I don't work for the company, but, like Jared of Subway fame, I would be willing to do a few commercials. Anybody out there? Anybody reading this? YooHoo!)