The best preservative is an oil that mixes with glow fuel that gets spread over all parts of the engine while it's running. The only one I know of that does this is castor and it'll preserve an engine for decades. The downside is that it goes gummy over a period of months and needs to be freed up before running the engine again, particularly in the wrist pin to rod area and the bearings. It's not hard to do by flooding the engine with acetone or fuel though. Other than that, auto trans fluid (ATF) is very good as is Marvel Air Tool Oil (NOT Marvel Mystery Oil). The ARO in small bottles from the LHS is auto trans oil. For long term storage you have to make sure that whatever you use gets to the front bearing because that's the hardest part for oil to get to.