Lubricity of compressor oils: achieved by "building" a base oil stock with naturally high film strength. That is the starting point and the reason for synthetics, to control properties by controlling the construction of chemical chains. The construction of a a synthetic oil from alcohol and acid, etc is quite a feat, at least to my way of thinking. The diesters and PAO's qualify as true synthetics in this regard. Beyond high quality base stocks, the oil manufacturer may elect to add certain additives. These have various functions but the additives which improve the film strength appear to be most relevant from the perspective of the compressor operator. WRT use in breathing air compressors, manufacturers of compressor oils have been cautious in adding chemical modifiers as these may be regarded as potentially toxic. Apparently, very few companies are willing to garner liability exposure or to undertake the expensive tests required to approve the additives. Hence, their use may be limited in oils commonly recommended for breathing air applications.
Currently, I have been recommending that users of Kidde compressors take a look at Exxon/Mobil Jet II turbine oil. This stuff is loaded with additives but nonetheless has passed a number of stringent tests for toxicity. Mobil has been gun shy on the subject and put a lot of pressure on the additive manufacturers to produce additives which would meet criteria even though the turbine oil is in no way intended for use in breathing compressors. Currently, the manufacturer of the Kidde recommends this oil based on its ability to protect their high speed machines.
Omnilube 455, a food grade oil, also contains additives to combat wear, corrosion, etc, yet is approved for use in machinery used to process foodstuffs. This is impressive as Omnilube has passed some tough wear tests including the Falex "three ball" wear test designed to test high pressure lubricants. This oil has a moderately high flash point. More importantly for some, it is said to be two-three times as resistant to oxidation compared to diesters when exposed to pure oxygen (rotary bomb test).
It appears that those compressors manufacturers who are recommending the 40W synthetics are basing this on wear and toxicity tests for extreme use. A 40W oil, being thicker, will normally have a higher film strength than a thinner oil. Moreover, responding to reports of toxic gases being produced by diester oils subject to high temperature it was likely felt that some protection from this problem could be achieved by transitioning to a different class of oils with heavier viscosity and more complex chemical chains. These are called triesters and are represented by oils such as Ultrachem Chemlube 800. Equally strong solvent properties but also stronger anti wear and with more stability is the story on this.
