When it comes to the oil that circulates through your engine, there is a lot of confusion floating around on the internet. Discussions about synthetic versus conventional, which viscosity is needed for a given application, and how often it needs to be changed are probably the three most frequently discussed subjects surrounding diesel oil. So, to get some insight into these topics, the folks at AMSOIL were gracious enough to clear the air.

The Big Debate About Which Is Better

Before we compare conventional and synthetic oils, we first need some insight into each. There is a lot that goes into creating both conventional and synthetic oils, but for the sake of this discussion, we’ll keep it simple. Conventional engine oil is made from crude petroleum that is pulled from the earth and then undergoes several refining processes to remove impurities. Conventional oil mainly consists of hydrocarbons (oxygenated or non-oxygenated), but it may also contain traces of compounds such as sulfur or nitrogen. After processing, additives are added to improve its performance.

Synthetic oil is made using pure, uniform chemicals that are blended to create benefits conventional motor oils can’t offer. There is a lot that goes into a synthetic oil and it can get tricky to classify a 100-percent synthetic oil. So, to get a better understanding of what makes a true synthetic oil, check out this article put together by AMSOIL. Now, back to the debate.

In no uncertain terms, synthetic oils offer improved wear protection, engine cleanliness, cold flow, and resistance to viscosity loss over conventional oils. That translates into a longer-lasting, higher-performing engine. If you are looking for proof, the American Automobile Association (AAA), recently published an in-depth report that affirms synthetics outperform conventional oil.

Diesel Oil: Weight Matters

Selecting the correct viscosity oil for your rig is easy. Read the owner’s manual or look at the fill cap. Asking what viscosity is best for your engine on a social media page is not a great way to get a correct — or definitive answer. What you do need to keep in mind is that the climate in which you operate your truck and the way you use your truck plays a huge role in what viscosity oil you use. All owner’s manuals will have recommended oil viscosity requirements for the climate conditions. For instance, if you live in a northern climate that regularly sees temperatures get cold for long periods, you will want a multi-viscosity oil with a lower first number (5W-30 vs. 15W-30 as an example). Study your owner’s manual and make sure you are using oil of the approved viscosity for the weather and load conditions you experience. Once you know exactly which viscosity oil you need, here’s a guide to the various diesel oil that is available from AMSOIL.

When and Why To Change Your Diesel Oil

First and foremost, just because your oil is black, does not mean it needs changed. The oil in your engine naturally darkens due to heat cycles, and the additives in the oil hold contaminants in suspension and prevent them from adhering to engine parts, which can turn the oil black. For gas-powered trucks, the rule of thumb is the oil should be changed every 3,000 miles. But that’s not necessarily the case for our trucks with diesel engines. There is no easy answer for when your truck will need an oil change, as there are many factors that contribute to the requirement: where you drive (like climate and terrain), what you do with your vehicle (such as often hauling heavy loads or towing a trailer), and whether you choose conventional, synthetic blend, or full synthetic oil.

Let’s say you only use your truck for short trips around town and rarely reach high speeds. If so, you might need to change the oil of your diesel-powered vehicle more often than a diesel-powered vehicle that routinely travels at highway speeds. This is because at lower speeds and temperatures, the oil in a diesel engine accumulates contaminants more quickly than if the vehicle were driven at higher speeds and in hotter weather.

A simple test of your engine’s oil will let you know if all is well or trouble is brewing.

Many diesel truck manufacturers recommend longer (up to 10,000 miles), thanks to oil-life monitoring systems. If you are using premium synthetic motor oil, like AMSOIL Signature Series Synthetic Motor Oil, it can last up to 25,000 miles (40,000 km) or one year. But, if you plan to use any oil for an extended period of time, sample testing is a great way to ensure your oil is still doing its job.

Passing A Test

Testing your oil is the best way to ensure your engine oil is still protecting your engine. Many diesel owners will test their truck’s oil at various mileage intervals (2,500, or 5,000) without changing it to learn the condition of the oil. An oil analysis will determine the condition of the lubricant itself as well as the engine or other lubricated components it interacts with. The information you get can tell you whether your oil is ready to be changed or if it can be left in service.

To have your oil analyzed, simply take a sample of the oil and ship it to a qualified laboratory. There, it is subjected to a range of tests to determine the concentration of wear metals, fuel dilution, the lubricant’s total base number (TBN), oxidation, and other information. The lab will return a report on the lubricant’s condition and include a brief explanation and recommendations for future service.