Castor oil will do nothing to displace the water and tends to gum up if it sits too long. I agree with the airtool oil or trans fluid. Transmission fluid is one of the best oils we have. What I do at the end of the day is run the engine out of fuel at idle, then give it a good squirt of WD40- the "WD" stands for water displacing which is a good thing, then run a few drops of air tool oil or trans fluid through it and you should be good to go.
WD-40 is the worst oil you could use. The solvent in it will dry out and the oil left over will harden, not soft like castor but very hard and will not easily dissolve. Yes castor congeals (notgum up), but it easily dissolves with fuel ormethanol and will even soften up. Also you don't want an oil thatdisplaceswater andas with all oils is lighter than water.That means it willfloat on top of thewaterwith the water sitting on the metal. Castor willdissolve with water to some extent andit doesn't evaporate or slide off of the metalinstead protecting it with itsvarnish. Iwill use 3 in one or motor oil for long term storage but only when sure the fuel has evaporated and there is no moisture present,but engines I have neglected to oil have survived decadesas well, protected by that castor oil.