There is a degree of ambiguity about how long motor oil actually lasts. Traditionally vehicle owners have felt more comfortable with convenient mileage or time-based schedules while others prefer seasonal changes, but the question still remains – how long does engine oil last?
Many factors contribute to the degradation of motor oil. Extreme heat breaks down oil molecules, which evaporate and leave deposits on internal engine surfaces, inhibiting the transfer of engine heat to the oil. Oxidation, which is accelerated by heat, further inhibits heat transfer. Environmental contaminants such as dust and dirt enter the engine through improperly maintained filters, and normal engine wear produces metallic particles that travel through the engine increasing wear. Byproducts of normal combustion – soot, dirt, and sludge – contaminate the oil and change viscosity. Finally, internal pressure breaks down the oil film between moving parts, which agitate the oil, trapping and suspending air and promoting oxidation.
Lubricant manufacturers continually research and experiment with various chemical additives that in some cases comprise 20 percent of a typical multi-grade oil. In addition to viscosity index improvements, which are the primary additives that allow for multi-grade oils, manufacturers use rust and corrosion inhibitors to neutralize acidic oxidation of the oil. Manufacturers use detergents and dispersants to minimize sludge buildup, along with anti-foaming agents and pressure additives to prevent oil barrier breakdown between internal engine parts. This is known as the shear factor. Manufacturers use additives to aid the base oil in the protection of engine components, but each has limitations and can be affected by the same factors that degrade base oils.
So there is no easy answer. Keep in mind the type of oil you are using, how hard you are driving your car, and just where you’re vehicle is being driven. When in doubt, stick to your owner’s manual.