Change it, you must

When to drain motor oil varies by engine and type of service, but it’s probably longer than you realize.

November 2020

Tom Berg


Changing the oil in your truck’s engine is like taking a shower. You come away cleansed, refreshed and just feeling good because you’ve done the right thing. But if you drain and refill engine oil every 12,000 to 15,000 miles, it’s like taking three or four showers a day. Neither is necessary.

We’ll leave your showering habits to you, but we have some advice from the people who design and build your engine and professionals who operate and maintain them: Find an optimum drain interval, and follow it faithfully because it will save you money and still keep the engine healthy.

Modern heavy-truck diesels are built to go 25,000 to 50,000 miles between oil changes and sometimes more, say their manufacturers.

They establish exact recommendations based on the type of service, or duty cycle, and how much fuel the engine burns, expressed in miles per gallon. The higher your mpg, the longer your motor oil can last – another reason to drive economically. The engine’s oil capacity also enters into their calculations, with more quarts providing a buffer against degradation from contaminants, buildup of acids and consumption of additives.

As to duty cycle, line-hauling is the easiest operation for an engine and qualifies for the longest intervals. Various vocational operations are harder on engines because there’s a lot of stop-and-start driving, and the truck is exposed to dirty and dusty conditions, such as trash hauling and construction. Also considered are operating environments, as in western deserts vs. wet climates, where air is regularly washed by rain; extreme temperatures, either very hot or bitter cold; extended idling, as during deliveries and overnighting; and varying loads, says the Technology & Maintenance Council in its Recommended Practice 334C, “Guidelines for Establishing Proper Engine Oil and Oil Filter Intervals.”

Basic intervals of 25,000 to 35,000 miles are predicated on normal operations and proper filtration as well as use of approved motor oils. Classifications have changed more frequently in recent years as petroleum engineers strive to design formulations that protect evolutionary engine designs. Low-RPM cruising, for instance, can put special stresses on internal parts and require upgraded oil to compensate.

The American Petroleum Institute’s classifications for diesel motor oil have included CJ-4, CK-4 and CI-4 (C for compression-ignition, 4 for four-stroke and the letter denoting progressively advanced formulation).

Which does your engine require? Go to your manufacturer’s website to see. While there you can look up a recommended interval for your engine model. The basic interval is the easiest to follow and will provide peace of mind, even if it’s costing you more money than if you use an approved extended interval. Whichever you pick, follow it to stay within the engine warranty’s requirements.

RP-334C includes a table that illustrates potential monetary savings due to going from a 15,000-mile interval to a 45,000-mile interval, for a tractor that runs 135,000 miles per year. Extending the interval reduces the number of oil changes from nine to three, and lists the savings in oil and filters bought and, for a fleet, a reduction in manpower in the company’s shop. Most owner-operators get their oil changed at dealer or truck-stop shops, which charge fees to reflect time, materials and labor. A typical oil-change service costs $300, so right there expenses are reduced from $2,700 to $900. What could you do with an extra $1,900 in your pocket each year?

Do-it-yourselfers can discount their own labor, but remember that time is money and might be better spent with the family.

But the DIY guy also has to deal with buying new oil and disposing of drain oil, as well as buying and discarding filters. There are different ways to properly handle waste oil and filters, but they all represent time, money and possible messes. (For nature’s sake, don’t bury the drain oil or dump it down a sewer!) The fewer times the oil is changed, the less is the expense of dealing with the materials. Yes, while under the truck and the hood, a guy can inspect the vehicle for other items that might need attention, but nothing says he or she can’t do that anyway, and as often as one’s mental state dictates.

Extended drain intervals, to 60,000 miles or more, can be done in some operations but require use of oil analysis. This involves periodically taking a small oil sample and sending it to a lab. There technicians run it through tests to find the amount of metals, acids and other nasties and send back a report. If it says the oil is still safe as defined by the engine builder and oil maker, then it can stay in the crankcase. Sometimes additives have to be replenished and filters must be changed, but most of the oil can be retained. Oil analysis is not expensive and a number of manufacturers and labs offer the service.

Theoretically, oil itself does not wear out and can be used indefinitely. Back in the early 1980s we knew a guy who ran a small dump-truck fleet in Southern California, and he never changed his oil. But he regularly changed the filters, including a big bypass filter, whose element was tightly wrapped in paper toweling. The element held a gallon of oil, and he replaced the dirty element and its oil at about 25,000 miles. The fresh oil replenished the additives and the new element went to work. Regular analysis proved the oil, and his engines, stayed safe. LL