Recommended Practices covering all aspects of vehicle maintenance, including how to safely extend intervals, are available from theAmerican Trucking Associations Technology and Maintenance Councilin printed form or on CD. Photo: Technology and Maintenance Council

By Paul Abelson

The best tool for determining the proper drain interval for any engine is analyzing the oil. To start, select a reliable lab to do the analysis, and stay with it. Whether it's an oil company or independent testing facility, always use the same location. Lab procedures and conditions do vary. Contract with one source and order a supply of bottles, containers, and labels. This ensures consistent results that you can use for apples-to-apples comparisons.

You'll get a report that shows the amount of metals in parts-per-million (ppm) and Total Base Number (TBN), a measure of alkalinity, which for new oil can be anywhere from seven to 12 or more. The higher the TBN, the greater the ability to fight acid. TBN is often used to indicate how much life is left in oil. When it gets down to two or three, it's time to change even if wear metals are low. One rule of thumb is to track Total Acid Number (TAN), which measures acid content; when TAN exceeds TBN, it's time to change oil.

Each engine manufacturer has limits for how much of each wear metal is acceptable. I've grown up with 100 ppm of iron as its danger zone, but most samples range from the mid-30s to the high 80s. Iron indicates cylinder liner or valve train wear. High copper, lead, and tin show bearing wear. Chromium is from piston rings, and aluminum is from pistons. Silicon may be from dirt or form-in-place gaskets. By charting the amount of each, you can follow how much wear and tear an engine receives.

To figure out how to safely go beyond the manufacturer's recommendation, take a sample of new oil as a baseline. Then take samples at the recommended service intervals. They set the intervals conservatively to minimize warranty claims, so don't worry: You'll have plenty of time to change if a poor report comes back.

If the report is good, take your next samples and subsequent samples at further half-intervals. For example, if the recommendation is to drain oil every 10,000 miles, send a sample at 10,000 miles, and again at 15,000 miles, then at 20,000, and so forth.

Buy fittings that allow technicians to draw samples from internal oil galleries. You'll get a cleaner sample than one siphoned from the filler or dipstick tube.

Extending oil drain intervals doesn't mean you can extend chassis lube intervals, too. You may have to lube trucks somewhere between 5,000 to 15,000 miles, depending on vehicle design, type of grease, and the skill with which it's applied. Synthetic grease lasts longer than mineral-oil-based grease.

Also, squirting grease into a zerk fitting until you see some fresh grease emerge isn't good enough. Wipe away the old grease and grime, and pump until new grease emerges uniformly around the seal. For some fittings, especially steering and suspension, don't stop when the near-side seal is good. Purge the old grease from both sides.

For spring bushings and suspension parts, the question often arises about whether to add grease with the parts loaded (sitting on the wheels) or unloaded (with the chassis jacked and the wheels hanging free). Some fleets play it safe. They do both.

For a more technical discussion on extending drain intervals, check out the American Trucking Associations Technology and Maintenance Council's Recommended Practices (RP) 334 and 1403: “Guideline for Establishing Proper Oil Drain Intervals.” RP 1403 is for light- and medium-duty vehicles; RP 334 for heavy-duty diesels. Order copies by calling 703-838-1763.

— Paul Abelson ( is a former director of the Technology and Maintenance Council (TMC) of the American Trucking Associations, a board member of Truck Writers of North America, and active in the Society of Automotive Engineers. He is co-author of TMC's RP 334 and 1403 “Guideline for Establishing Proper Oil Drain Intervals.”

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What to do with lab data on aluminum, chromium, copper, iron, lead, silicon, and tin.

Don't just read the reports, chart them with a spreadsheet.

Plot parts-per-million (ppm) on the vertical axis and miles on the horizontal axis. Connect the dots and look for unusual trends. Soon you'll be able to project when readings will reach critical levels. If you decide on a 30,000-mile or longer drain interval, continue to analyze the oil every 5,000 miles.

If a line moves sharply up or down, change the oil even if ppm values are still acceptable.

Let's say you're tracking iron to an acceptable 100 ppm (see chart below). In this example, the spike from 45 ppm at 30,000 miles to 78 ppm at 35,000 miles is a sure sign that something isn't working properly. Change the oil at once, even though the 78 reading is within the 100 limit.

If the 35,000-mile reading goes down to 26 ppm, you've got a leak. (The fact that you've been adding oil frequently should have alerted you anyway.)

Do this for every wear metal. Charting the amount of each reveals engine wear and tear.