I found this item on the Lube-Tips website. It is referring to industrial lubricants, but the same is probably true for motor oils.
============================================
Q & A: Stored Oil vs. In-service Oil Life
"I heard that oil has a shelf life of one to two years, but oil in service can last for several years. How is this?"
What you have alluded to is true. Oils do have a fairly short shelf life compared to the length of time they can last in service, if properly maintained.
The issue here is additive solvency as it is commonly referred to. Chemically speaking, "additive solvency" is a bit of a misnomer, as many additives are mixed or suspended, rather than dissolved in the base oil. When the lubricant is stored for an extended time, the additives can settle out. This is particularly true of lubricants that have high additive treat rates, and leaving such oils sitting for extended periods can leave a thick sludge at the bottom of a drum. The problem is exacerbated by base stocks with a high nonpolar content, given that many additives are polar in nature.
Using the oil effectively keeps the additives well mixed. If you can limit particulate and moisture contamination to the minimum you will avoid particle scrubbing, water washing and other destructive depletion reactions. As a result, your oil will likely last for a substantially longer time than its shelf life.