I do flush through all my steam engines after steaming - first with WD40 - then with Motor oil. 30 years ago I worked on the introduction of UK produced Motor Oil for the car manufacturer I worked at. The local company was Texaco, and the chemist (PhD in oil!) explained how Motor oil is one of the highest quality oils for chemical protection of systems with mixed metals and non-metals, temperature and other chemical pollution. Due to its environment and service in a car/truck engine. Lubricity is just 1 of its 101 properties. The chemical package blended with the oil - mostly for anti-corrosion - is a company secret - always. So 6 months of lab and engine testing went on before a specific Texaco oil (Havoline) was approved... Since then I have always used Motor oil for metal protection - after de-watering.
WD40 is supposed to be acidic, so that as well as de-watering, it partially dissolves the rusts and oxides existing on metal surfaces. But this can cause long-term erosion in left as "anti-rust protection" - which is not what it was originally blended to do. But "Duck Oil" is an excellent water displacement and corrosion protective oil.
I would not trust either of these for lubricity - the initial lubrication after long-term storage is expected to be poor for these products, so please re-oil before running.
Never use brake fluid or other hydraulic fluid. - except where designed - in hydraulic systems.
Paraffin/Kerosene does not protect from Rusting. High quality DERV (diesel oil) does, because it contains special additive packages from the oil benders. But "cheap" diesel fuel does not have the anti-corrosive and initial lubricity from additives that exist in regular Branded Pump fuel (even stuff from Supermarket pumps).
Normally I would have recommended high quality coolant from a respected brand of car engine coolant supplier... but that was discounted earlier in this thread.. (Nerd1000 to propclock comments). Just one comment, Glycol added to water varies the freezing point of the coolant - but the Coolant supplier to the factory where I worked advised we install water treatment plant as the local water contained the "wrong minerals" and was likely to develop excessive sediment in the cooling system... - which chemical reactions would shorten the life of the anti-corrosion package they used in their coolant. So it's not Glycol protecting the engines from corrosion, but the additives in expensive coolants...
I don't know anything about machine coolants. Hence I am learning from your experiences.
K2