We would define it as the period following its manufacture during which a lubricant stored in its original container could be deemed suitable for use without a re-test of its physical characteristics prior to use.

Lubricants are inherently stable materials.  Neither synthetic nor high-quality petroleum oils would be expected to oxidize, polymerize or volatilize over any reasonable (decade-long) period at the usual temperatures of storage.  We have regularly checked the quality of certain synthetic hydrocarbon-based precision bearing oils over periods exceeding five years and have detected no significant change in viscosity or neutralization number, which would be the signal properties for degradation.

Hydrocarbons and silicones would likely be least affected by simple aging.  Ester oils, where the ester linkage may be subject to a minute degree of hydrolysis in the presence of moisture, could experience greater changes in neutralization number over long periods if moisture is present.

Greases can "age" in more complicated ways.  The most likely mode by which time could affect grease quality would be by contraction of the gel structure.  If this occurred, significant oil bleed would be evident and the remaining grease would stiffen.  In other greases, the gel structure changes in such a manner that the grease becomes softer over a period of time.

The oil separation or "puddling" which can sometimes be found on the top of the grease in bulk pails or drums is usually insignificant when the separated oil quantity is compared with the mass of grease present.  Such oils can normally be safely stirred back into the bulk of the grease in the container.  Experience has shown us that much of this time-related oil separation can be reduced if the surface of the grease is kept smooth during storage; i.e., don't leave "craters" in the grease.

Oil separation over time is also a function of grease consistency.  An NLGI Grade 000 grease could be expected to exhibit pronounced oil separation from the day of its manufacture.

As with oils, there are straightforward bench tests by which grease qualities can be measured and any "age-degradation" determined.  These are worked penetration, dropping point, oil separation, and, again, neutralization number.