Either ATF or Air Tool Oil, about six drops in the intake.  Flip a few times.  Place in a plastic bag with a paper towel around the engine.  For extra credit, add a drop on the crankshaft threads and run the nut over it, then brush a few drops over all the head and backplate bolts.

Getting the castor off an engine takes more than running out a prime.  Burned on castor will last for decades.

I'm in the process of storing most of my engines which have always been run with all castor fuels so some are quite gummed up. First step is flooding them with acetone to dissolve as much of the gummy castor as possible and making sure it gets to the bearings and that the top end of the rod is free to slide. Then I give them about a 5 minute 4 stroking run with a blend of methanol and 25% Mobil Jet Oil II which should clear out any remaining traces of old oil. The jet oil is like an EP version of ATF except it mixes with methanol. So far I've treated ringed engines, like my Dooling 29, lapped iron, ABC, ABN, AAC and Norvel AAO.

i try to make sure the motor is "run out" of fuel complete by starting it off a prime, then continue to flip it a bit until it stops the "farting".

remove glow plug, put a few drops of ATF or air tool oil down the plug hole, a few drops down the intake, and flip it over a bunch.  replace the plug, flip a couple more times, then plug the intake and exhaust with paper towl making sure the piston isn't anywhere near TDC.  this assumes it's in a plane.

if not, odds are it hasn't run in a while, or may have come off a wrecked bird.  same thing, but it goes in the box it came out of.

The idea of the plastic bag is to isolate the item from the outside air.  The paper towel around the engine cuts down on UV, also serving to absorb some of the moisture in the bag.

Some of the Japanese engine makers, especially Saito, do a really good job of oiling, bagging and boxing their engines.

That foam that Fox used is awful for storing engines.  Deteriorates, leaving the owner to try to get it off, hoping it's not sticky.

Plastic bags don't last forever, either, especially if the storage area is not temperature controlled.

Try Marvel Mystery Oil or Marver Air Tool Oil. While chemically identical, Mystery Oil has a pink tint that adds color and smell to the oil. Either one acceptable.

Use FoodSaver vacuum bags for each engine. It evacuates all air and moisture from the bag allowing the engine to be stored indefinitely. No rusting is possible.

   As Brett stated, regular Marvel Mystery Oil WILL congeal over time, at least as bad or worse than castor. I have been a victim of that mistake. ATF has detergents and rust inhibitors in it and is my first choice if I'm going to oil up and engine. Having worked in that business for ten years and seeing old tranny parts being refurbished and rebuilt, I don't recall ever seeing anything gummy. Vacuum bagging sounds good, but I would wrap the engine with paper towels to pad it since there are sharp edges and pointy spots that may poke through the bag. I don't know how thick they are. I wouldn't suck the bag down all the way either to help avoid that issue.
   Type at you later,
    Dan McEntee