Originally Posted by Tamer Bekir
I would never wait that late to get an oil change, around, near or passed 10k. I do not live in a dusty or dirty area either so 4k is too soon.
It may be possible to safely exceed even 10k mil OCIs. UOA showed my Audi's oil to still have 30% more miles under the same conditions left at 12k miles.
There is a lot more to oil "wearing out" than contamination by particulates. Dusty conditions are just one factor contributing to oil becoming unsuitable.
One can measure the TBN (Total Base Number), which indicates the reserve alkalinity remaining in the oil, which is necessary to neutralize the acidic byproducts of combustion. At some point the oil's acid neutralizing capacity is exhausted. TAN (Total Acid Number) measures oil oxidation and build-up of acidic compounds.
Oil also may thicken over time due to contamination. This can result in sludge and impeded lubrication and increased/abnormal wear.
Oil can also shear (get thinner) over time due to mechanically getting "beat up" (cams, timing chains, valve train), and the resulting thinner oil viscosity may cause wear under high stress conditions (for example, high oil temperature).
Oil also can thin due too fuel dilution (fuel gets into the oil due to incomplete combustion)
Moisture can accumulate in the oil (engine does not get fully warmed up/driven only short distances). This can also cause sludge buildup.
Anti-wear additives in the oil get over time depleted.
A modern car's OLM gives a good indication regarding oil life and is a good starting point. An occasional UOA will allow maximizing oil change intervals safely.