6 Best 15W-40 Oils for Diesel Engines (2023)

When it comes to buying a 15W-40 oil for your diesel engine, going cheap is the worst decision you can ever make. Owning a car or truck with a diesel engine is a costly venture and how long it serves you will depend on several factors including the quality of oil you maintain the engine with.

In this post, we’d be looking at some of the best 15W-40 oils on the market.

Best 15W-40 Oils for Diesel Engines

Best 15W-40 Oils for Diesel Engines: Our Top Picks

If you’re already figured that 15W-40 is the right viscosity grade for your diesel engine and that it meets the engine requirements, you would need to look towards purchasing from a high-quality brand to ensure that the oil does its job effectively.

In this piece, we have curated a list of the 6 best 15W-40 oils for diesel engines – carefully handpicked based on chemical composition, performance, brand reputation, and buyer reviews.

Now, let’s cut right to the chase!

1. Red Line RED21405 15W-40 Diesel Oil


If you’re looking for a top pick for your medium or heavy-duty diesel vehicle then the Red Line 15W-40 Diesel Oil is an option to try out. This engine oil is designed with top properties that will boost engine life.

It is designed to be highly resistant to thermal breakdown which ensures maximum protection. That’s to say, if your car gets too hot, due to hot weather or heavy-duty use, this engine oil will give your car all the protection it needs to ensure that your engine doesn’t get damaged.

This engine oil also provides double the detergent and acid-neutralizing ability of conventional diesel engine oils for maximum engine safety and cleanliness. So if you’ve got a diesel engine that runs high oil temperature or you operate it in hot weather or you pull heavy loads with it, then this oil would give you all the protection you would need. It provides excellent value for long-term fleet use while boosting the durability of your engine to the maximum.

Alongside all these, this oil contains polyester base stocks which will withstand the heat of modern jet engines and also provide superior drain intervals. This synthetic oil also features a natural multi-grade property which allows the manufacturer to avoid bulking up the oil with unnecessary additive packages, so you get something that is effective and efficient without being too much for your vehicle.

Overall, the Red Line RED21405 15W-40 Diesel Oil offers top-notch protection and the lowest friction in your diesel engine has been proven to be true which is one of the reasons it comes out on top in many lists. If you’re going for an option that will serve you well and keep your engine running smoothly, then this engine oil is an option you wouldn’t want to overlook.

2. Royal Purple ROY04154 Oil


The Royal Purple ROY04154 Oil is a top diesel motor oil filled with premium-based oils and additive technologies to ensure that your vehicle’s engine gets a high-quality boost in performance. This engine oil is designed with a bit more features than regular engine oil and it doesn’t rack up too much in costs making it a great choice for those on a budget.

With the Royal Purple ROY04154 Oil in your vehicle, you get superior corrosion protection that would keep your engine running smoothly and efficiently even with prolonged use. The enhanced additive technology in this pick prevents metal-to-metal contact while the low coefficient of friction boosts fuel efficiency.

These features also knock out the competition as they are so much more than what you would expect from other brands, however, this brand doesn’t stop there with its offer to ensure you get superb performance and all-round protection.

The patented additive technology will prevent lubrication starvation and white sludge which, in turn, prevents the occurrence of rusts.  The oil is designed to be generally compatible with fuel containing ethanol. It is also a universal fit type which makes sense as it doubles as a gasoline engine oil giving you protection in both vehicle-types.

The Royal Purple ROY04154 Oil is actually a synthetic oil. But due to its high degree of natural solvency, it’s able to prevent engine problems by removing deposits of previously used oils and maintaining overall engine cleanliness.

While the Royal Purple ROY04154 Oil comes with many pros that will boost your engine performance, there are quite a few things to watch out for with this oil. One of the most important is its container which might not be rated correctly.

You might notice that a gallon bottle may contain fewer ounces than advertised. Luckily, this is only possible if you buy from the wrong source. Another thing to take note of is the requirements. Before using this oil, your vehicle would have to meet certain mileage requirements for the best performance.

3. Shell Rotella T5 Synthetic Blend 15W-40 Diesel Motor Oil


If you own a medium-heavy-duty diesel vehicle with a high requirement for engine durability, then the Shell Rotella T5 could make for the best pick. This oil comes with advanced properties including Triple Protection Plus technology which boosts its protection ability.

Starting off, you’ve got the Triple Protection Plus technology which is a unique combination of advanced additive technology and synthetic base oils. This feature helps to provide superb protection against wear, deposits, oil breakdown, and better low-temperature flow. You can rest assured that the oil will keep your car engine protected from damage if it gets too hot during use.

Shell Rotella T5 is synthetic blend oil is engineered to provide excellent engine equipment, prolong engine life, and maintain viscosity control even under high temperature. This oil is designed to work with a wide range of heavy-duty engine applications so if you own a diesel-engine vehicle that relies on increased heat and stress resistance, then this oil could get the job done. It provides just the same protection you would expect from an SAE 15W-40 but in a synthetic blend engine oil.

With Shell Rotella T5, you can be sure of up to 37% better wear protection than the max wear requirements. It is also designed to provide excellent piston cleanliness exceeding the limits of some top car manufacturers.

In addition to that is an advanced low-ash formulation that helps to control the blocking of exhaust after-treatment while boosting engine fuel efficiency and helping maintain vehicle emission compliance and engine fuel efficiency.

Regardless of what you’ve heard of synthetic base oils, the Shell Rotella T5 is one that stands out. Its synthetic base oil components provide the ability to improve cold starting and low-temperature performance while reducing fuel consumption.

4. Lucas SAE 15W-40 Magnum High TBN CI-4 Oil


The Lucas SAE 15W-40 Magnum High TBN CI-4 Oil is another top pick that you can never go wrong with, however, this superb diesel engine oil is vehicle specific. This means it would only deliver superb performance in certain vehicle types.

In compatible vehicle types, the performance of the Lucas CI-4 Oil can hardly be matched thanks to its properties and features that ensure it not only keeps your engine running smoothly but also prolongs your engine’s lifespan.

The Lucas CI-4 Oil was designed to deliver twice the lifespan of plain 15W-40 oils which ensures you spend less and get better protection than with these regular oil types. It will hold much more soot in suspension than conventional 15W-40 oil which keeps your engine clean.

This is a big plus as such particles could cause big problems for your engine if left unchecked. This is just one of the ways this oil is built to boost your engine’s fuel efficiency and lifespan.

With this engine oil, you can also expect extended drain intervals and wouldn’t have to worry about dry starts.

It also ranks as a top lubricant for hard-working hydrostatic transmissions. The Lucas CI-4 Oil is designed for use in pre-2007 trucks without diesel filters. You should ensure that your vehicle meets this requirement before purchasing this oil so you get the best of experiences with it. It is also engineered to be very effective in engines using high sulfur fuels.

5. Mobil Delvac 15W-40 1300 Super Motor Oil


If you’re on the market for a top-notch oil brand for your diesel engine that won’t poke a hole in your pocket, then the Mobil Delvac 15W-40 1300 Super Motor Oil might be your best pick.

This oil offers excellent soot-viscosity control that enables greater engine efficiency while boosting engine life. It also offers superb thermal and oxidation stability which will reduce engine deposits and sludge build-up. For you, this means your engine will run more reliably than it did before.

This oil also comes packed with enhanced TBN reserves which offer excellent corrosion protection and extend drain intervals for all engines – old and new using either low (500ppm) or very-low (15ppm) sulfur diesel fuel.

Another well-thought-out feature is the low-temperature pumpability along with the excellent corrosive and abrasive wear resistance. These features are designed to provide greater wear protection which will prolong engine life. It also offers excellent oil consumption control so you won’t have to use up too much oil during operation thereby slashing your overall oil costs.

This oil is also designed to meet the demanding specifications of original equipment manufacturers (OEMs) and the top API gasoline services category. You can use this oil for mixed fleet operations so you won’t have to spend extra to get the right pick.

Overall, the Mobil Delvac 15W-40 1300 Super Motor Oil tops as an option for shoppers looking for something with excellent quality that would keep their diesel engine vehicle running efficiently.

6. Lucas Oil 10299 SAE 15W-40 CJ-4 Synthetic Motor Oil


If you’re on the market for synthetic motor oil with superb cleaning properties, then you should look towards the Lucas CJ-4. This oil comes blended with premium Lucas additives that offer a lot more protection than you would get from regular motor oils. These additives help to maintain the life of emission control systems.

You should also expect less oil consumption, as well as, improved engine life all thanks to this oil. The low oil consumption comes due to the improved properties.

With other motor oils, the contamination with combustion by-products and sometimes wear would cause you to change them. With the Lucas CJ-4, you won’t have to do so very often. This oil features high detergent action that allows it to suspend twice as much contamination and still provide good lubrication qualities.

It is also engineered to resist oxidation and thermal breakdown during operation to ensure that your engine remains protected whether it is old or new. The final perk with going for this pick is that the oil is dual-purpose meaning it would work for your diesel engine as well as your gasoline engine.