Quote:
Originally Posted by cRiPpLe_rOoStEr
This may explain in part the high oil consumption of some engines, as some broken-down hydrocarbon chains might become much more volatile. Remember the so-called catalytic cracking of kerosene, resorted by some refineries to make high-octane gasoline?
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From what I understand it's the viscosity index modifiers that break down first and cause it to become thinner. Basically 5W-30 becomes 5W-20, 5W-15 and eventually just plane 5W once the viscosity index modifiers have broken down. Then as contaminates fill the oil it starts to eventually thicken again.
The base stock oil is the last to break down and change. If it changed too much it wouldn't be recyclable.