Oils & Fluids
Oils can be subdivided and taking each in turn we can compare their merits as lubricating oils:-
Vegetable Oils are cheap and biodegradable however they are naturally variable in quality, there is a limited range and they are unsuitable for extreme temperatures. They are also chemically unstable.
Mineral Oils (hydrocarbon) are derived from crude oil. They are relatively cheap, have moderate chemical stability, they are available in many different grades however they are also unsuitable for extreme temperatures. They are also less biodegradable.
Polyalphaolefins , PAOs—synthetically produced Mineral oil without the impurities, higher temperature resistance, less lacquer and can use the same additives as Mineral oil.
Poly alkeylene Glycols, PAGs—Can be oil & water soluble or neither, they have a very low lacquer, higher temperature resistance however additives are generally less effective
Esters, Diesters, Triesters & Polyol Esters
Higher temperature resistance than PAGs or PAOs however they also more expensive. Polyol Esters are more expensive than Triesters which are more expensive than Diesters. Temperature capability rises with price. Polyol Esters are also biodegradable.
Synthetic, Non Hydrocarbon oils—these are generally Silicones, Fluorosilicones and Fluorinated oils or PFPEs.
Silicones have a wide operating temperature range and are generally compatible with most non silicone based materials. They are expensive.
Fluorosilicones have an even wider temperature range and are even more expensive.
Fluorocarbons (PFPEs) - far and away the ultimate performance oil with respect to chemical & thermal stability with the widest operating temperature range, they are compatible with almost all materials and resistant to all factors that degrade most oils. They are the most expensive however the performance is unequalled in most applications.