Most everyone knows you need to change your oil in your car, and even relatively inexperienced truck owners well know you need to change the oil in a truck diesel. Fortunately, the oils used in truck diesels are extremely advanced, and the capacity of the oil system is much greater than what you have in your car, even considering the size of the engine and how hard it works. This means oil lasts much longer in a truck tractor diesel engine, though it still very definitely needs to be changed at the right interval.

But, why does oil have to be changed at all?

An Internal Combustion Engine

The biggest reason is that a diesel is an internal combustion engine. In a steam engine, where the fuel is burned in a boiler furnace, the working parts are never contacted by the burning or burned fuel and exhaust. Oil in power stations with steam turbines lasts many years.

But, in an internal combustion engine, the fuel is burned inside the machine in close proximity to moving parts that are lubricated by the oil. This means that the oil is subject to something called "blowby" where the oil is actually directly exposed to partially burned fuel and exhaust, as well as high heat from metal parts like the pistons and valves that are directly exposed to the searing heat of combustion.

Lubrication and Cooling

The result is that the oil must lubricate the engine and withstand forces that tend to shear or rub across it, and help to cool parts like pistons and valves, which exposes it to temperatures in the range of at least 300 degrees. The process of protecting the engine also will force soot, imperfectly burned fuel, water formed in combustion, and acids that result when diesel fuel burns in a metal cylinder right into the liquid lubricant. One reason this happens is that some of the engine's exhaust and unburnt fuel actually flows through the crankcase, the housing for the crankshaft and connecting rods.  The oil is stored in a big pan, a sort of tank that's open at the top, that's attached to the bottom of that crankcase.

Fortunately, truck engines hold a large volume of oil -- much more than a car. Crankcase capacity including what flows through the filters is typically 45-55 quarts, and the oil's temperature is also effectively limited by a large oil cooler‚ a big enclosed radiator or heat exchanger cooled by the engine's cooling system.

Filtration

Highly effective filtration, usually including an especially fine filter not used in car oil filters, typically is included in a diesel engine's lubrication system. For example, on Volvo and Mack trucks there are two large full flow filters, and a separate filter so fine that only part of the oil stream passes through it, though all the oil will run through this filter several times an hour. Other engines combine these two slightly different filtration functions into a single, very large filter assembly that may hold a couple gallons of oil.

Diesel Engine Oil Technology

Diesel engine oil is an extremely high-tech product that periodically undergoes significant improvement. The diesel engine oil standards are revised every 5-8 years or so, and the improvements in the product are quite amazing. Today's oils withstand the attack of high heat, acids, etc. far better than oils of the past, and a battery of tests is required before they are approved by a large organization called The American Society of Testing and Materials. Oil is typically developed by engineers with a PhD in chemistry -- it's that complicated.

Diesel Engine Oil Properties

The oil in a diesel engine must keep soot in suspension so the oil won't get gritty or even too thick, also suspend other impurities so they won't create clogs or varnish, deal with small amounts of unburnt fuel that may leak in, keep water that results from combustion from thinning it out, and neutralize acids. It must also withstand the heat it's exposed to without losing its ability to lubricate, breaking down into a thin, watery substance that could not lubricate the engine. Lubrication actually means enabling the engine’s parts to float across one another so the metal surfaces, which are rough on a microscopic level, will not touch.

How Often Do You Have to Change Your Oil?

Tradition-bound truckers for years would change their engine oil at 15,000 miles. However, change intervals have been lengthened by many factors in recent years, including vastly  improved oils, better diesel fuel injection systems (because soot and other products of unburnt fuel really foul the oil), the removal of nearly all sulfur in 2007 (which then made it much easier to protect the oil from acids), and engine improvements. For example, Cummins, a company that sells engines to all the truck makers, improved its piston in 2017 so that much more of the heat it absorbs from the burning fuel would pass directly into the engine's cooling system. A system which passes oil across the bottom of the piston to help cool it could then be reduced in capacity, not only reducing the size of the engine's oil pump and the amount of power needed to drive it, but subjecting the oil to considerably less heat. Changes like these have enabled engine makers to commonly lengthen their oil change intervals to 50,000 miles or even longer.

Factors Affecting Oil Change Intervals

How to determine when to change is a science, and so should be seen as much more than guesswork. The engine and truck manufacturers all publish standards based on miles driven, and, if you know how much fuel you burn per mile, and approximately how much of the time it's running that your engine is just idling, you can make an educated guess as to when you should change. These facts will not only help you decide when to change, they can guide you toward saving money and downtime by minimizing idle time, maximizing fuel economy, and extending change intervals.

Manufacturers’ Recommendations

Most charts showing manufacturers’ recommendations are divided into three categories: Severe Duty, Normal Duty, and Light Duty. Light Duty normally means 6.5 mpg or more, easily obtainable with late model equipment with aerodynamic chassis features when geared for fuel economy.

Idling

Idling may also be part of the calculation in determining the recommended change interval, with high idle times contributing to significantly more frequent changes. You may be puzzled by this because changes are determined in large part by fuel consumption and idling doesn't use all that much, maybe a gallon per hour. But, the problem is that, even though injection and combustion systems have improved a great deal in recent years, the fuel just does not burn as cleanly or completely at idle.

Piston Rings

Also, piston rings, which work best when subjected to high pressure, seal less efficiently at idle, allowing the products of incomplete combustion to pass into the crankcase and contaminate the oil. (This is what we are referring to when we talk about "blowby".) So, all smart operators find ways to cool and heat the cab and sleeper during long shutdowns with some sort of Auxiliary Power Unit, whether a small diesel, or some sort of battery storage system that runs a separate air conditioner when you are shut down overnight.

A Better Way - Laboratory Oil Analysis

There is a better way to determine oil change intervals than even careful use of a multi-faceted chart, and that is laboratory oil analysis. You contact a lab to get a sample bottle, take a sample of the engine oil as it is draining, and then mail it into the lab.

How Oil Analysis Works

The lab, in a manner similar to a blood test, uses sophisticated instruments to test the oil for things like soot levels, acidity, TBN which is an additive system that combats acid, wear metals of all kinds, and viscosity, or how thick or thin the oil may be compared with its original characteristics. Because of its precision and sophistication, analysis can do a better job of telling you when you really need to change. Some manufacturers actually approve extremely long changes up to 70,000 or even 80,000 miles if the truck is run efficiently, you don't idle, and the analysis report shows the oil is still protecting the engine. So coordinate with the engine manufacturer's recommendations to see if they approve of longer intervals if analysis shows the oil is OK. This is especially significant if you have an extended warranty on your engine.

Since the report actually determines wear metals and can compare their levels to previous experience, taking several samples x-number of miles apart during the life of one oil change can measure the amount of each wear metal per mile to see how constant it is. You might find, for example, that the wear metals that come from bearings, cylinder liners, and other parts doubles when you extend your interval from 30,000 to 60,000 miles. That indicates the oil is still protecting the engine. If the amount of one or more metals had more than doubled with the longer interval, you would know you needed to shorten the change interval.

Analysis will normally allow you to extend your change interval if your engine is in good condition and your operating conditions favorable. Make sure to get filters that will last the life of an extended change, or to change them before draining the entire lube system if the interval is so long, it exceeds the life of even the best available filters.

Other Benefits of Oil Analysis

Analysis not only helps you determine when changes are really needed, thus saving oil, filters, and downtime, it will warn you if something has started to cause excess wear. Small leaks of diesel fuel or antifreeze into the oil will immediately start to show up as excess wear. With analysis, you can frequently solve the problem before it causes a significant amount of engine damage. If a part were starting to fail, the report would also show excess wear metals and tip you off to the need for the repair. The lab can identify which part or parts are likely to be failing because they know which engine parts are made of each type of metal. It can also ID the culprit like fuel or antifreeze from the chemistry of the oil.

How Do You Get an Oil Analysis for Your Diesel Engine?

You can easily do an Internet search to find oil analysis labs. Search "heavy-duty diesel engine oil analysis". Laboratories like Titan Laboratories, Blackstone Laboratories, or JG Lubricant Services will analyze your oil. Also, Baldwin, a famous filter manufacturer, sells oil analysis. You typically purchase a kit for about $20 that includes a small bottle that you mail in to the lab after catching some oil as it drains from the engine. The price includes lab analysis and a full report. Make sure you purchase a premium report that includes all wear metals, viscosity (thickness) at two temperatures, any contaminants like fuel or antifreeze or water, and TBN and TAN. These last two, total base number and total acid number, determine how much longer the oil will protect your engine from acids that can be generated in various ways and that will soften engine metals and cause severe wear.

Oil companies like Chevron, Shell, and ExxonMobil will also offer oil analysis and you may be able to get it free or at reduced cost through your oil distributor if you consistently buy the same oil. Engine manufacturer Cummins provides CumminsOilGuard. Check with your engine manufacturer as well as other sources to see if you can get analysis at a low cost or even free of charge so you can get the best deal.

What Oil Should You Use for Your Truck?

It's also smart to buy the very best oil, as this will not only help the engine to live longer, it normally means you can extend the drain interval, saving money on oil and filters, and downtime. Some companies work with a particular engine maker to formulate an oil especially good at protecting an individual engine brand. This works because different engines not only are made of slightly different materials, but the character of the combustion may be different, causing the levels needed for various protective additives to be adjusted. Once you find the ideal oil, and have extended your intervals to the optimum mileage, you will be able to save a lot of money and perhaps even have some bragging rights.  And, if the types of routes you travel, the traffic and weather conditions change, make sure to watch for the need to shorten your intervals, or even an opportunity to make them longer.

We can discuss things like oil grades and viscosity in a future article. The latest oils CK-4 and FA-4, are intended to save fuel, and FA-4, is particularly efficient at lubricating your engine while generating minimal friction, thus saving fuel. However, it is too thin to be used in any but the latest engines. You may also want to choose a lighter viscosity oil, or a heavier one, depending on the kind of duty your truck sees, the climate where you operate, and even the season. We'll cover some of these subjects in later articles.