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A key property of the oil is its oxidation stability – that is, its ability to resist oxidation.
A standard measure of this property is the Rotating Pressure Vessel Oxidation Test (RPVOT, formerly RBOT, ASTM D 2272). The test presents a simulated worst case scenario, where an oil sample is subjected to the harsh conditions of high temperature and high pressure oxygen atmosphere in the presence of water and a copper catalyst. As oil oxidises oxygen is consumed and the pressure drops. Time taken to achieve a specific drop in pressure is then recorded.
By monitoring the RPVOT value of a lubricant its remaining useful life can be assessed. Note however, that while this test was very useful for assessing Group I oils, which age in a fairly predictable manner, with Group II oils things get a little trickier. Many of the modern synthetic lubricants achieve more than 1000 minutes in the RPVOT test. However, for some oils this is the result of the additives inhibiting the copper catalyst. As a result, the RPVOT value measured will stay very high until the additives have depleted, at which point it may reduce rapidly.
It is therefore prudent to augment the RPVOT test by also monitoring levels of the anti-oxidant additives. For that we can rely on the RULER test.