Oil Change: What is the best maintenance interval?

By Jim Marotta


The 3,000 mile oil and filter change is as much a part of the American car culture as hot rods, car hops and whitewall tires. Whether we perform it ourselves in the driveway or watch while the family “mechanic” puts the car up on the lift and drains the oil, we think of the 3,000 mile number as the standard interval for oil change service. But, look at a current maintenance interval chart; manufacturers are gradually extending service intervals.

What is the best interval for an oil change? Manufacturers are extending service intervals. Photo by Jim Marotta.

The move toward extended service intervals coincides with new oil technologies that allow manufacturers and refiners to create more stable oils that will endure longer service, and a decrease in the frequency of other services such as tune-ups.

Conventional vs. Synthetic


Traditionally, manufacturers made conventional engine oils from a mineral base oil stock derived from crude oil. Synthetic base oil stocks are produced by either highly processing and refining mineral based oil stock, or by chemically engineering synthetic base oil stock. Current additive packages blended into conventional oils allow them to function longer than oils of the past, but it is primarily the synthetic base stocks that allow greatly extended intervals.

Synthetic oil offers many advantages in durability versus conventional petroleum oil and it is all based on the synthetic oil’s superior resistance to heat and oxidation while requiring little or no viscosity index improvements which are the most vulnerable to degradation as oil ages. Synthetic oil is more accurately controlled at the molecular level and therefore can be engineered to resist the forces in an engine that work against oil durability.

How Long Does Engine Oil Last?


Clearly 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.


Chemical additives can comprise 20 percent of a typical multi-grade oil. Photo by Jim Marotta.

How Do I Know When My Oil Needs To Be Changed?


One solution presented by the automobile manufacturers is an oil life monitor. Several types of systems exist that use technology to directly “read” the oil. The most popular of these systems rely on capacitance. Using two conducting plates separated by the oil, which acts as an insulating fluid, the system calculates a base capacitance number. As the oil degrades, the system measures the change in capacitance.

Other systems use algorithms to predict oil change intervals.The algorithms are based primarily on factors such as driving habits, driving speed, and failure to replenish low oil levels. Research determined that these factors contribute to oil breakdown. The algorithm method systems also monitor time between oil changes, vehicle speed, coolant temperature, load signal, engine rpm, engine oil temperature and engine oil level to determine the remaining time and mileage before the next oil change. General Motors' Oil Life System and Chrysler Corporation’s Flexible Service System (FSS) are good examples of this type of system.

General Motors’ Oil Life System is based on algorithms that attempt to predict oil change intervals. Photo courtesy of GM.

Goodbye Dipstick?


A new multifunctional system determines oil level and oil condition. The system determines viscosity using a piezoelectric listening device. When this sensitive surface comes into contact with the oil, the oscillation frequency and damping change according to the oil’s mechanical properties, allowing viscosity to be measured. The addition of oil level sensing capability may allow vehicle manufacturers to remove the oil dipstick from the engine.


So, the best answer to our question of how long engine oil lasts is, “it all depends.” Follow the manufacturer’s fluid specifications and maintenance intervals when servicing your vehicle. Get them at www.ChiltonDIY.com or www.ChiltonPRO.com, where a valuable subscription to comprehensive service procedures, specifications, and wiring diagrams, gives you all the information to do the job right.

A muscle car enthusiast and drag racer, Jim Marotta is a freelance automotive writer with more than 20 years experience in the automotive industry.


Posted 16 Feb 2011 6:05 PM by Jim Marotta