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If you have been reading this column over the years, you know that the subject of when to change your engine oil has been mentioned on many occasions. Its a question that both passenger car and heavy-duty truck owners struggle with still.

In the old days, I tended to be cautious, believing that an oil change was cheap insurance for my car. More recently I have come to rely on my engines oil life monitor to alert me when a change is needed. That said, I want to take a look at the minimums, maximums and everything in between of oil changes.

At one end of the spectrum is dont ever change the oil. Strange as it may seem, there are a few advocates of this position. One quote from a website talking about why oil changes are being extended reads, I plan to trade my car in as soon as I reach the end of the 36,000-mile warranty so I put synthetic oil in at the first change and Ill never change it again! For me, thats a bit risky. However, it has been pretty well documented that people who lease their vehicles are, shall we say, a bit less careful about maintenance.

Historically, there was one well-known, essentially no change recommendation. It was by Detroit Diesel in the old days of high oil consuming two-cycle engines. The companys basic position was that as long as oil make-ups were done regularly, operators didnt need to change for 100,000 miles. At the rate those engines consumed oil – about 250 miles per quart – and needed topping up, you were essentially making a complete oil change every 8,000 to 12,000 miles. Those were the good old days for oil marketers!

Widening the Gap

Speaking of the good old days, the 3,000-mile/three-month (3K/3) oil change interval was not too bad either. Starting in the late 1960s, 3,000-mile intervals were recommended for any driving regime. However, that changed in the next decades. As engine design and fuel delivery systems improved and oil quality was upgraded, drain intervals were extended for engines seeing normal service. But the 3K/3 recommendation was maintained for severe service.

Then as now, severe service was defined in owners manuals as stop-and-go driving, dusty environments, towing and a myriad of other conditions. This severe definition essentially covered almost everything, so 3K/3 was still the dominant recommendation. In the original equipment manufacturers mind it was a cautious position – one that minimized potential warranty claims.

Dick Kabel, former engine oil guru with General Motors Research, shared some information with me regarding oil change intervals up to 1985. It clearly shows that the OEMs (at least GM) were concerned about engine durability and felt that 3K/3 was best. When GM introduced its Oil Life System, the need to define a specific oil change interval was reduced. The first system was in the 1985 Corvette; by 2003, it had been incorporated into virtually all GM vehicles.

Last October, my column noted that drain intervals for the various passenger car sellers had lengthened. Drain intervals ranged from15,000 miles down to 5,000 miles. Clearly, the OEMs have made an effort to extend oil drains. Their motivation for such a change is to reduce the amount of used oil in the environment and to recognize the actual practices going on in the field. Not coincidentally, the quality of engine oils has improved appreciably from 1986, when the highest engine oil category was API SF/CC, to 2009 where engine oil is ILSAC GF-4/API SM. Without these upgrades, longer drains would not be possible.

There are also some oils in the marketplace which promote even longer drains for the average automobile. There are so-called long drain oils such as Mobil Clean 5000 and 7500, which have been developed with the express purpose of capitalizing on the whole long drain equals superior quality idea. There are also some specialty synthetic oils such as the Amsoil line, which claims drain intervals up to 25,000 miles.

Bottom line: For the driver who wants them, there are a lot of passenger car engine oils out there making long-drain claims of varying intervals.

Big Engines, Big Decisions

The area where the drain interval has the most importance, in my view, is the heavy-duty engine oil market. Major engine builders such as Volvo/Mack, Cummins, Detroit Diesel, Navistar, John Deere and Caterpillar all have recommendations for drain intervals. However, they are all based among other things on application, duty cycle, customer needs and seasonality.

So what is a good truck fleet to do? Fortunately, there is an answer and it is called used oil analysis.

Over the years, oil analysis has developed from tests for viscosity, flash point, fire point, color, pour point, carbon residue and corrosion (which were all part of the 1923 SAE Viscosity Classification, pre-J300) to include tests such as metals analysis, total base number, total acid number, infrared analysis and more. And thats for new oils!

All of these tests plus others for water, soot, antifreeze and wear metals, plus others for oxidation, nitration and other degradation are employed for used oils. With such a formidable arsenal of analytical weapons, it seems likely that no error of nature or man can go undetected. So then, the question becomes: When do we change? What are the indicators that the oil has reached its useful end? In short, how do we go about setting condemnation limits?

The answer to that lies in field testing and in monitoring fleet operations. When new engines or lubricants are developed, a lot of field testing is conducted to determine how they will perform in the real world. Field tests are typically set up to evaluate engines and/or lubricants under severe conditions. The two most important parameters for a field test to demonstrate performance are either high mileage (hours) or severe operations (loading or climatic conditions).

In addition, existing fleets can be monitored to determine oil performance over a long period of time. These operations provide a strong platform for engine oil proof-of-performance information. Some fleet tests are specific to a new engine design, such as the introduction of Exhaust Gas Recirculation (EGR) or Selective Catalytic Reduction (SCR) systems. These tests usually run on prototype oils.

The Base Number Puzzle

From all of these tests and the accumulated data, limits for engine oils are established. Limits are based on used oil analysis and include information on viscosity increase, total base number (TBN), wear metals and contaminants. Instrumental techniques can also measure the extent of nitration and/or oxidation that the oil has experienced.

The limits vary by engine manufacturer and fleet operator. However, there are some general numbers that appear to be common to everyone. The most common is viscosity, or rather viscosity change from the new oil. In most cases, a viscosity change by one SAE grade (kinematic viscosity at 100 degrees C) is cause to change oil. (Of course, it isnt always as simple as that; some engine builders note the change as a percent.)

Almost every heavy-duty engine builder has an absolute minimum TBN for the oil to be used. TBN indicates the total base components remaining in the oil to combat the buildup of harmful acids, and typically should measure at least 2 to 3, per ASTM test method D4739. Since current engine oil formulations have new oil TBN values of around 10, it is obvious that theres a lot of TBN lost before the oil is unfit for use.

Remember that there are different TBN test methods, which give varying results. The old ASTM D664 test method typically gives lower test values due to the fact that it measures only the so-called hard TBN supplied by the metallic components in the oil. Detergent components such as neutral and overbased sulfonates, phenates and salicylates contain levels of colloidal metallic carbonates (usually calcium or magnesium) which are measured by this method.

However, there are other components in the oil – such as dispersants – which also provide base but which are not measured by ASTM D664. Newer test methods such as ASTM D4739 as well as ASTM D2896 also gauge the basic content of these additive components, which results in higher overall TBN values.

Other oil quality parameters which vary by equipment manufacturer include wear metals, such as iron, lead, copper, chromium, tin and aluminum, and contaminants. The presence of iron, chromium and aluminum in the oil sample is the result of wear in the pistons, cylinder liners and rings. Other contaminants (including silicon, soot/solids, sodium, fuel dilution, potassium and boron) come from various outside sources such as dirt, engine coolants and exhaust blowby. All of these potential nasties can wreak havoc with an engine.

Oil analysis also measures the levels of additive metals. The most common of these are calcium, magnesium, and zinc. Molybdenum has also become a metal associated with the additive package. When compared to new oil, the additive metals actually can show some increase, as they become concentrated due to volatility losses in the crankcase. Use of these parameters enables the fleet to determine if the right oil is being used in the vehicle.

Analyzing the Options

There are several laboratories in North America which specialize in used oil analysis services for on- and off-road equipment. Most major oil marketers offer used oil analysis programs as well, although many of them subcontract the used oil analysis labs to do the analytical work.

These services are a valuable part of fleet maintenance but they are not free. Fleet owners should carefully weigh the cost of the service versus the value it brings to their operations. Are there enough vehicles to warrant the service, or is it better to simply drain oil on a predetermined schedule? In that case, shorter oil drains will probably be safer. Is the vehicle fleet sufficiently valuable to justify used oil analysis? Large-capacity ore haulers in a mine would be a good example of a fleet that could benefit from the service.

Setting up an oil analysis program requires a lot of work to get samples at regular intervals. First, the lab will need a sample of the new oil for comparison. After that, used oil sample results can be reported accurately. The lab will undoubtedly have a reporting system which will provide quick turnaround on samples and have a system to flag problems. E-mail, telephone or fax reports are the most commonly used.

For those results which indicate satisfactory oil condition, with no evidence of system degradation, a mailed report is acceptable. If the program has done a good job of tracking the oil condition in various units, it can be expected to identify trends before they result in an engine failure.

So the discussion continues: When do we drain the oil in an engine? Is it by mileage or by oil condition? If by mileage, what is the proper value? Is it better to drain sooner, and add additional used oil to the environment? Or drain later and increase the possibility of premature engine failure? If we use oil condition to determine drain intervals, is the cost-to-benefit justified?

Some things just dont have easy answers.

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