Oil goes bad
One main difference, is oil in an engine has an unlimited supply of new oxygen coming in contact with it, where as oil in an unopened container doesn’t. This oxygen interacts with oil additives, oxidizing them, or “using them up”. Oil with depleted additives no longer does its job of not only lubricating, but cleaning. There are also additives that counteract or neutralize the acids formed from combustion products. These combustion products interact with water to form the acids. These acids can corrode the components inside the engine. The water in the air, and the higher the humidity of the region the engine is in the more water there is in the air, the more water there is available to interact with the oil to make acids. These are some of the main reasons oil is recommended to be changed at six month intervals when inside the engine, whether the engine is run or not. Oil stored in an original unopened container, on the other hand, is good for many years…
Jim Baker
RV-6
N699JB
CFI-IA