This echos my opinion, but; I will toss out CLP as another low cost easy to apply chemical. Kinda split the difference where oil is a bit thick and WD40 thin and too volatile for my liking. You can buy CLP in gallons now, although I dont use enough for that. I did find Lucas offers a CLP, so; for me that leaves a lot of BreakFree brand to use up.Unless you are trying to preserve original finish or markings, the one pictured has already suffered through the ravages of time and the elements....As stated, it appears stable so a minimal amount of maintenance is all that would be required....You might want to just daub on some cheap motor oil or spray on some WD-40 periodically....This will leave a coat on it and give it a darkened appearance....Bodes
Normal CLP's are good for a year and BreakFree now has a 5 year formula. If that matters.
I do have a couple big basket ball size cannon balls on my damp cellar floor. No coating at all. They seem like some kind of iron that does not rust like steel in the same environment. Nothing maybe all that is really needed. Or CLP my 2c.