Can You Run 15w40 in a Gas Engine? (Read This First!)

15w-40 engine oil is a premium fleet engine oil which is specially formulated for providing reliable protection for heavy vehicles like buses or trucks by using Compressed Natural Gas (CNG) or Liquefied Natural Gas (LNG). 

Generally, high-quality base stocks and a well-balanced additive system are used to formulate such engine oils. 15w50 oils are mostly used in diesel engines because of their much different additives profile. 

Due to the use of high antiwear additives like ZDDP, such oils have slow wear on engines. Now, let us look into whether 15w 40 can be run in a gas engine or not.

Can you run 15w40 in a gas engine?

If the oil meets the engine’s viscosity requirements and other necessary specifications like API SN or API SP, then such gas engines can be run using high viscosity diesel oil like 15w40. However, such diesel oil is not needed. Instead, 5w30 is the best option considering value and performance.

Walk through this section and learn a bit regarding 15w40 –

Rotella 15w40:

Shell Rotella T4 15w40 engine oil is a conventional diesel engine oil that flexes a triple protection technology against oil breakdown, deposits, and wears. 

It has been formulated with advanced additive technology that provides enhanced deposit control and superior engine cleanliness spreading throughout the oil drain interval. 

It claims to have enhanced protection against oil oxidation for more efficient operation. It also has increased shear stability for better viscosity control and optimal engine oil pressure. 

The intensive wear control helps the metal engine surfaces keep moving for longer life. Rotella T4 also meets and exceeds API CK-4 requirements, and it is designed in a way that it can provide extra performance in both new and old engines. 

It weighs around 7.73 pounds with a dimension of 12.75*9.5*13.5 inches. It comes in a single 1-gallon bottle with an ergonomic design which is easy for handling and pouring. 

It takes pride in providing exceptional protection for longer life in a wide range of heavy-duty engine applications, starting from tractors to diesel pickup trucks, to tractors and tractor-trailers. 

The company claims to have used a proprietary formula that is exclusive to shell while developing this product. 

It is primarily designed to provide protection under the most severe engine conditions, especially found in modern-day low-emission engines and older hard-working diesel engines. 

The improved oil aeration also results in heightened durability of the emission control system. All these claims have been made in comparison to previous generation CJ-4 products. 

What is 15w40 oil used for?

As the 15w40 engine oil is a heavy-duty diesel engine oil, it can be used successfully in off-highway diesel equipment, over-the-road diesel trucks, turbo-charged diesel engines, farm tractors, passenger cars, light trucks with diesel.

You can use them even in gasoline engines that are backwards compatible with previous API oil categories.

It is safe to use in gasoline engines only when the diesel oil meets the proper specifications and viscosity requirements of your engine. Since such oil has a much different additive profile and very high antiwear additives like ZDDP, it leads the engine to wear very slowly. 

This is why mostly heavy-duty engines are the primary choice for the use of 15w40 diesel oils. But such engine oils are less effective in cold temperatures. 

The winter viscosity number dictates the cold start performance, so less thick engine oils might outperform 15w40. Even 10w40 will have a better cold start performance than 15w40 given its thickness in cold temperatures. 

Will 15w40 hurt a gas engine?

If 15w40 engine oil is used instead of regular 5w30 oil in a gas engine, there will not be any substantial damage. But you will consume more fuel as the crank in the engine will be more loaded. 

Due to the increased viscosity, the oil will not blow off easily. So, the engine will be worn out faster than before as the oil will not flow to the moving parts quickly. If wearing down quickly is not an issue for the engine user, then using 15w40 will not be hurtful at all. 

But the most important factor is considering the appropriate specifications and viscosity requirements of your engine. Meeting such requirements eases the whole procedure and makes the use of such heavy-duty engine oil smooth in gas engines. 

It is to be kept in mind that wear protection should not be sacrificed for fuel economy. Engine oils such as 10w30 are thinner and less strong, hence they are more prone to oxidation. In this regard, the use of properly specified 15w40 might turn out to be beneficial instead of hurtful.

Is Rotella 15w40 good for gas engines? 

The Shell Rotella T4 NG Plus 15w40 engine oil is a scintillating choice for mixed fleet gas engines. It meets a wide range of specifications and can be used instead of natural gas engine oil, heavy-duty diesel engine oil, and passenger car motor oil. 

Specification-matched Rotella oils can be used in gas engines even as they are providing diesel engine protection. The Shell Rotella T6 multi-vehicle 5W-30 was specifically made for gas engines as well, providing a varied range of options.

But specifically using the 15w40 will have varied results in gas engines. The viscosities of 10w40 and the 15w40 are the same, but the temperature difference while functioning will be greater.

The viscosity number in winter determines the cold start performance in that season. That is why the 10w40 will have a better cold start than the 15w40 in gas engines, even though it is good in a general sense. 

3 reasons why you can/cannot run 15w40 in a gas engine

Walk through this section and learn why you can/cannot run 15w40 in a gas engine –

Fits Necessary Requirements:

You can run 15w40 in a gas engine mostly if it can fit the necessary requirements like API-CK4, CJ-4, CI-4 Plus, CH-4 and similar benchmarks that determine whether viscosity intensive engine oils can be properly used in gas engines or not.

Using such oils will not pose any harm to oil engines and will provide wear protection as well while it is being used. These oils, like the Rotella T4 triple protection 15w40 engine oil, have extended specification support that includes gas engines as well. 

Hence, such oils can be used in gas engines while the required specifications are being matched. 

The Climate Factor:

The climate is also a necessary factor that should be considered while determining whether you can run 15w40 in a gas engine. Given that places with a cold temperature will require a better cold start performance throughout the year.

In this case, 15w40 is not the best choice as it has better alternatives that have more suited temperature control in cold weather. But in normal/warm environments, 15w40 will have a better and long-lasting performance. 

So, the requirement needs to be considered while concluding.  

Use Of Unspecified Diesel Engine Oil:

If unspecified 15w40 diesel engine oils are used in gas engines, then there is a possibility that the fuel filters, as well as the fuel injectors, will be clogged. That will cause great harm to the engine. 

So, in cases when specification matched oil is not available, 15w40 engine oils cannot be used in gas engines. 

What happens when you run 15w40 oil in a gas engine?

The following scenario may occur if you run 15w40 oil in a gas engine –

Increases Fuel Consumption:

Using 15w40 engine oils in gas engines substantially increases fuel consumption. This is because of the increased crank load that occurs because of using an engine oil with more viscosity. 

So, if the expense is an issue, you might want to switch to the regular 5w30 gas engine oils which might provide less wear protection but are more budget-friendly and fits all other requirements. 

Wears Down Engine Faster:

As the 15w40 oil will not blow off easily, it is very likely to wear out your engine a lot faster than gasoline motor oils that have great cold start properties like 5w30. 

Occasionally Clogs Filter & Injector:

Diesel oils are a lot denser and thicker than gasoline oil. Hence, it does not flow through the moving parts as quickly as required. There remains a slight possibility of the fuel filter and the fuel injectors ending up clogged. 

In that case, the 15w40 cannot be used in such gas engines. 

Final Thoughts 

High viscosity diesel oil like 15w40 should only be used in gas engines if the oil meets necessary specifications like API SN/ API SP and is aligned with the viscosity requirements as well. However, there are better-suited alternatives like the 5w30 that work fine considering value and performance.