There's little parts and pieces of metal in that oil and carbon buildup also. So molecules actually crack and break apart with temperature in all substances. Just sitting here, ammonia nitrate fertilizer will deteriorate. Then we put it out in UV light and that UV light is absorbed by it too and breaks it apart by the absorption of protons of light. It's hard to get microbes to eat the longer chains.
Nature breaks it down for the microbes to eat in the soil.
You can take a plastic milk jug and set it out on the ground. The sunlight makes it brittle and crack, and the wind just breaks it into little parts. Eventually microbes eat it up. Nature is always making big things small. You have a thermometer, get ice water and put a thermometer in it.