Diesel Oil in Gas Engine-Will It Hurt My Engine?
In 1993, when the EPA began cutting back on the acceptable levels of ZDDP (Zinc and Phosphorous) in engine oils due to the damage it causes to catalytic convertors, a big hole was created in its wake- gasoline engine oil for older vehicles still needed the higher ZDDP levels. It took about 10-15 years for this to become a realization in the automotive world once people started having longevity problems in their older cars. The first solution was the idea of using diesel oil in gasoline engines, as diesel engine oil was not subject to the new ZDDP rules. This held fast until the engine oil industry began to make ZDDP additives and high-zinc content oils for older gas engines and rebuilds where ZDDP is very necessary. For more on the benefits of ZDDP, check out our article on the benefits of zinc in oil.
Shell Rotella Diesel oil is the #1 brand most mechanics and hobbyists recommended; however, the rules have changed on diesel oil too, so is it still OK to use it in a gas engine? Do diesel oils have Zinc or ZDDP in them anymore? If you have asked these questions or didn’t realize you needed to be asking this question, you are in the right place.
Diesel vs Gasoline Engine Oil
You may think that oil is oil, and while the base oil is the same, the rest of the formulation is much difference between the two types. The reasoning behind this is the mechanisms in play in both types of engines. A diesel uses its fuel to make power much differently from gas engines. A diesel uses glow plugs to help run the engine, but the glow plugs are only there as an assistant. Once running, a diesel engine relies on compressing the air in the chamber so much that it heats up to around 550 degrees C, and then high-pressure diesel fuel is injected into the chamber, and the heated air ignites the fuel. The burn is unregulated and uncontrolled, temperatures exceed 2500 degrees C. A diesel needs 210 degrees to ignite, but diesel is a combustible liquid; the liquid itself burns, not the vapors. If you have tried to light diesel on fire, you know how hard it is to get it going. Unlike a gas engine, the burn rate of a diesel engine can vary wildly from stroke to stroke because there is no ignitor; the compression of the air itself is the ignitor. Diesel is nothing like gasoline, and burning diesel generates different byproducts than gasoline.
Gasoline engines operate very differently. The air-fuel mixture is compressed, but instead of relying on heat from pressure to ignite the fuel, a spark plug lights the fuel at the optimum point in the piston stroke, making this a controlled burn. Gasoline is a flammable liquid, it has a high vapor pressure. Unlike Diesel, it is the vapor of gasoline that burns, which is why gasoline is so much easier to light. A tiny spark can set off gasoline vapors, even though the ignition temperature is about twice as high as diesel. Pull the lid off a gas can, and you can see the vapors pushing out of the nozzle; this is the part that burns.
Because the mode of ignition and burn is so different, the byproducts of combustion are different. Diesel engines generate a lot of soot, whereas gasoline engines do not generate soot at all. Diesel engines produce a significant amount of particulates, hence the addition of DPF filters in newer diesel engines. These particulates can be up to 40 times more than gasoline engines while producing less carbon dioxide. Gasoline engines produce really dirty exhaust, which is why gasoline cars have catalytic converters to help clean it before it is released into the atmosphere. Because of these differences, the detergency levels in diesel oils are entirely different from gas engine oil. It is the catalytic converter that has led to the reduction of Zinc in gasoline engine oil and the reason that some people suggest using diesel oil in gas engines.
The byproducts necessitate the differences. Diesel oil additives are formulated to grab onto soot and other byproducts at a much higher level, meaning there are more detergents in diesel oil. Gasoline oil has to meet a delicate balance between cleaning and scrubbing parts from buildup and reducing the level of detergency in the oil that is inevitably burned in the combustion chamber through blow-by.
The viscosity ranges for diesel and gas engines also vary widely. Diesel engines need heavier-weight oils to protect the internals from shock from combustion forces. Oil molecules can shear under pressure, which lowers the oil’s lubricity, leaving the internals unprotected. Gasoline engines have tighter tolerances on the bearings, requiring lower viscosity oils than those used in diesel engines. Diesel oil is typically 15w-40 or higher; only older, worn-out gas engines should run 15w-40. Speedway Motors offers a selection of Motor Oils. You can use 10w30 diesel oils in a gas engine, as well as 5w40 diesel oils in a gas engine, but most heavy-duty diesel oils are higher viscosities.
Does Diesel Oil have Zinc?
Diesel oil does have ZDDP; however, the reduction of ZDDP has hit diesel oils as well. Using Shell Rotella diesel oil in a gas engine has become commonplace, as it has a CJ-4 API rating and has about 1200 PPM of ZDDP, which is enough to support a flat-tappet camshaft engine that has been properly broken in, but this is only part of the story. Every oil manufacturer has different formulations, Shell Rotella oil zinc content is about 1200 ppm, but other manufacturers limit the content based on the API ratings.
The rating of the oil plays a massive role in the decision of whether or not to use it in your engine. All diesel and gasoline engine oils carry an API rating, which starts with S for Service for gasoline engines, while diesel oils are rated starting with C for Commercial. Some oils are dual-rated, carrying both S and C ratings, for example, CK-4/SN. This oil would be appropriate for 2016 and newer diesel engines and 2015 and newer gasoline engines, regardless of fuel type, as long as the viscosity matches the engine’s required weight.
Dual-rated oils, such as the above example, are limited to the ZDDP levels of the gasoline oil rating, so a CK-4/SN oil would be limited to 800ppm, which is not enough ZDDP to protect a flat-tappet camshaft engine. This is an issue inside the diesel engine community and led to the development of Ford diesel oil. All newer Ford diesels are required to use Ford-approved oil, not just the same API rating. GM has a similar rating, with the 2020-newer GM diesels requiring dexosD-rated oil, not just CJ-4 API-rated oil. These oils have the correct level of ZDDP of 1200 PPM.
There is one more piece to this puzzle. ZDDP comes in two flavors- primary and secondary. Primary ZDDP (ZDDP1) activates at higher temperatures and provides less anti-wear protection but has a low volatile phosphorous content. This is what diesel oils are formulated with. Gasoline engine oils use both primary and secondary ZDDP (ZDDP2). Secondary ZDDP is for cold conditions and carries higher anti-wear lubricity but has more volatile phosphorous. Gasoline engines need the secondary ZDDP for more protection from cold-engine start-ups than diesel engines.
Can Diesel Oil Hurt a Gasoline Engine?
Beyond the API ratings, is there a difference between diesel and gasoline engine oil? The answer is an emphatic yes; there is a big difference. Using oil such as Shell Rotella diesel oil in a gas engine is a risky proposition because diesel oil has so much more detergent. It can remove factory-applied coatings on internal components that are meant to seal the engine’s combustion gases. This leads to more oil (and detergents) in the exhaust, which will damage the catalytic converter. In addition to the detergency and additives, the viscosities of diesel oil are higher than your typical gas engine.
Gasoline engines burn very hot, with average combustion temperatures of 4500 degrees F, and as hot as 6000 degrees inside the chamber possible. Diesel engines burn much cooler, around 2800 degrees F. This is critical for the oil formulation because diesel oil has much higher levels of additives and detergents, and gas engines burn hotter than diesels, you might see where the problems lie—gasoline engines burn off more oil, and the included additives into the exhaust. This will eventually burn up your catalytic converter. If your vehicle is older than 1975 and does not have a catalytic converter, then the differences won’t really affect much, but the main issue is the catalytic converters and, of course, emissions testing.
The best option for a gasoline engine that needs a higher ZDDP level is an appropriate gasoline oil in the correct viscosity and higher ZDDP content or a standard SN API-rated oil and a bottle of ZDDP additive.