Oil is a combination of many hydrocarbon chemicals, which the industrial chemists in the oil companies try to make as stable as possible.
However, many of these chemicals - especially the additives - are designed to react rapidly to the presence of undesirable chemicals that the oil comes into contact with - particularly, water.
If water in any form comes into contact with the oil, then the additives designed to deal with and neutralise water (and the byproducts of water - such as rust), will start to activate and thereby reduce in quantity.
Any moisture introduced into engine oil compartments will start to react with any bare metal surfaces it can find, and any number of compounds in the oil, that are a by-product of combustion.
Thus, we have sulphides, nitrides and other undesirable compounds in the oil that can form sulphuric acid and nitric acid when water reacts with them.
When this happens, the other additives that are designed to neutralise acids start to react with these compounds, and are gradually "used up", or contaminated by them.
Moisture gets into engines that are laid up, via condensation - especially if you get wide temperature variations between night and day in your area, or if your local climate is hot and humid.
Condensation and water that has not been in reaction with the oil is driven off by heating, when the engine is run up to operating temperature.
So the best thing to do, to avoid oil degradation and engine corrosion problems when a machine isn't being used, is to run it up to operating temperature every few weeks.
This runup will also spread oil around on splash-fed surfaces that need the protection of an oil film.
Oil very gradually drips off camshaft lobes and gears and other surfaces that need protection, and it's important that an occasional runup puts an oil coating back on them.
Oil companies often put out recommendations for oil changes based on calendar time, set to accommodate the very worst conditions that they can envisage.
Of course, it's helping their sales levels if they advise changing your oil every few months, even when the tractor hasn't been used.
