There was a time, not so very long ago, when the oil change recommendation was every 3,000 miles or every three months, whichever came first. It was also common for the recommended oil viscosity to be 10W-30 or 20W-40. Since the 1990s, this has changed.
Today, most manufacturers recommend 5W-30 oil viscosity. Lower viscosity oil requires less energy to pump it through the engine and creates less viscous drag internally in the engine. Viscous drag is the resistance the oil presents to such rotating members as the crank shaft and cam shaft. By reducing the energy lost in pumping and circulating the oil, the efficiency of the engine is increased and, therefore, the mpg of the vehicle is improved.
Today’s modern oils do a better job of lubricating. This is evidenced by the internal wear observed when tearing down an engine for rebuild. “Back in the day” when a mechanic was disassembling an engine he would find a wear pattern in the cylinders that would leave a ridge at the top of the cylinder. This is where the top piston ring would stop during normal operation; the ring would wear the cylinder wall forming a ridge where the wall was untouched. The piston could not be removed until the ridge was machined off, usually with a tool called a “ridge reamer”.
Today the mechanic will typically find a buildup of carbon that can be easily removed with a scrapper; there will be almost no wear observed. The modern oils are doing their job magnificently.
The less viscous oils do a better job of getting into the nooks and crannies of the engine. They also assist in cold starting the engine by presenting less viscous drag and they lubricate the cold engine more efficiently.
The next item to consider is “shear”. To understand shear, consider two flat steel plates with an oil film sandwiched between them. When sliding the plates apart the oil film is experiencing shear. Shear is what wears out oil and oil companies add modifiers to the oil to combat shear. This improved oil chemistry is at the root of the increased longevity of modern oils.
Finally, oil filtration systems have been improved over the years. Oil filters do a better job of trapping carbon and dirt, extending the life of the oil in the crankcase.
All of these advancements, in addition to synthetic oils, have served to extend the life of the oil in the engine. Most manufacturers recommend approximately 7,500 miles between oil changes. Several have added sensors to detect oil condition and a message will appear on the instrument panel when oil change time has arrived. Others have generated a computer algorithm that considers the mileage and the manner in which the vehicle has been driven to determine when to set the oil change message. The bottom line is the old saw of oil changes every 3,000 miles no longer applies.
The driver should not be fooled by the appearance of the oil on the dipstick. Modern oils are detergent; they pick up and retain dirt and carbon until it is removed by the filter or when the oil is changed. Dirty appearing oil is doing its job by holding the dirt in suspension. Oil should be checked when the engine is warm and the oil has cycled through the filter. Dirt in the oil is not what is of concern, but rather the oils chemistry. Old oil has experienced shear and the modifiers have worn out and that is what is behind the need for a change.
Armed with this information it should now be understood that extended oil change periods are normal today. The natural resource that is oil can be extended and preserved by following a more reasonable regimen.