Way back a little over 30 years ago, the automotive oil industry got flipped on its head. Emissions standards evolved to the point that the lubricating oil used in gasoline engines needed to change. The issue was that catalytic converters were plugging up too fast, and once that happens, they stop functioning. Studies found that the culprit was excessive ZDDP (Zinc DialkylDithioPhosphate), more commonly referred to as simply zinc. Most oils carried about 1,200 ppm of ZDDP, but modern engines didn’t need that much, so new oil ratings were introduced, lowering the zinc levels to about 800 ppm. However, that wasn’t the case for diesel oil.

For the modern engines, this made no difference, but as many a forlorn gearhead found out, the old flat-tappet cams really need that extra ZDDP, especially in the first 1,000 miles of use. A freshly built small-block Chevy with flat-tappet lifters would wipe out the cam lobes during the first 30 minutes of break-in, and it took a minute for the industry to sus out the issue — not enough zinc. The interim solution was to use diesel engine oil, which still had 1,200-plus ppm of ZDDP. This solution worked until high-ZDDP engine oils were reintroduced for older engines. Unfortunately, the whole “diesel-engine-oil-in-gas-engines” stuck like Oklahoma red clay on a white truck — it ain’t ever going away.

You may be thinking, “Well, oil is oil as long as it is the same weight, right?” The answer is both yes and no. The base oil can be the same, irrespective of the type of engine it is going into, however, the additives are where they diverge. Gasoline and diesel engines operate in very different ways, each requiring its own special formulations. Gas engines burn fuel through spark ignition, where diesel engines don’t have any sort of spark; they operate through “autoignition” via compression.

One of the foremost experts on engine oil is Lake Speed (The Motor Oil Geek on YouTube), who is a certified tribologist and more passionate about oil than almost anyone else we know. He recently released a video comparing diesel engine oils to gasoline oils and how they function in the opposite engines. If you want to get deeper into the chemistry of engine oil, Lake Speed delivers.

What Is Compression Ignition?

Diesel fuel systems use two pumps: a lift pump to move fuel from the tank to the engine and a high-pressure mechanical pump that pressurizes the diesel fuel to as high as 30,000 psi. Once injected into the combustion chamber, it is further compressed by the piston, which heats the diesel to the point that it spontaneously combusts. This is the point of the autoignition. The injectors inject diesel fuel multiple times during the compression stroke, near the top of the stroke (TDC). There is very little time for the fuel to vaporize, which allows pockets of dense fuel (not atomized with air) to form in the combustion chamber. When that dense fuel burns, it turns to soot. While most of it gets blown out the exhaust, some soot is drawn into the crankcase, contaminating the oil.

Once the engine is running, the heat of the chamber itself helps sustain the burn. This is an unregulated, uncontrolled ignition event. Because of how they operate, every stroke of the piston burns a different amount of diesel, leaving more byproducts in the chamber, mainly soot. Diesel soot is incredibly sticky and causes all kinds of problems inside the engine.

What Is Spark Ignition?

Gasoline engines use spark plugs to initiate the combustion process. When the spark plug fires, the fuel burn is instantaneous. Gasoline combusts violently, whereas diesel has a slower burn rate. Because the ignition and burn of gasoline occurs in a precise manner at a precise time, every single time, it is a controlled ignition event.

The process of combustion changes the byproducts of the fuel burn. Diesel generates a lot of soot; gasoline doesn’t leave soot at all. So, what does all of this have to do with motor oil, which is not in the combustion chamber? One word — blowby.

All engines have blowby, it is unavoidable. For diesel engines, that means that a lot of soot finds its way into the engine oil. Motor oil’s primary function is to lubricate the moving parts, but it has to do so under varying conditions. This means that the oil must function when cold, hot, dirty, and contaminated. Detergents are added to the formula to combat these issues in each type of engine. Soot is tough to clean, so diesel oils have substantially higher percentages of detergents (three to four times more) compared to gasoline oils. That’s where the problems start.

The higher concentrations of detergents in the oil become hostile inside a gas engine. Diesel oil can strip away protective coatings from pistons, valves, gears, and other items inside a gasoline engine, reducing the seal. This means more blowby and more of those chemicals getting into the exhaust, killing the catalytic converters.

Another issue that many people don’t think about is the fact that diesel engines run much cooler than gas engines. Gasoline combustion can reach 4,500 degrees Fahrenheit, whereas diesel combustion comes in at 2,800 degrees. This greatly reduces the overall temperature inside the engine itself. This is what allows diesel oil to have such high concentrations of detergents, because they are not exposed to temperatures where they become most volatile. Put that same concentration in a gas engine, and boom, you have more byproducts from burning detergents not only in the combustion chamber but also contaminating the oil as well. These byproducts turn into acid when condensed into the oil, eat away at the bearings, metals, and seals in the engine.

What About ZDDP?

While it is true that the diesel oils have higher levels of ZDDP, that is only half the story. There are two types of ZDDP, primary (ZDDP1 or Z1) and secondary (ZDDP2 or Z2). While the components of each type are the same, it is how the molecules are formed that changes how they act on metals. Motor oil loses viscosity at 260 degrees, however, the rotating assembly and valvetrain temperatures operate at over 400 degrees (at the point of contact). This is where primary ZDDP is activated, forming a sacrificial layer between two metals, protecting moving parts when the temperatures are too high for the base oil itself to do the job. Primary ZDDP is best for sliding surfaces, such as bearings and flat-tappet lifters.

Secondary ZDDP (Z2) is the opposite, as it protects parts in cold conditions, such as cold starts, before there is any oil pressure. Gas engines see higher RPM and more stress than diesel, making Z2 more important. Additionally, Z2 is less stable at higher temperatures, so it breaks down faster. Diesel oils only contain ZDDP1 because they operate with lower stress and experience fewer cold starts. Gasoline engines need both Z1 and Z2. If you run a diesel oil in your gas engine, you are missing a lot of protection.

This infographic from the Petroleum Quality Institute of America not only breaks down the API service categories as they relate to model-year, but also how they interrelate to other specifications, such as ILSAC and General Motors.

ZDDP is not the holy grail many think it is; you can have too much. Zinc levels above 1,500 ppm can have the opposite effect inside your gasoline engine. This is because at higher concentrations, it becomes corrosive, eating away at the metal inside your engine, taking off years of life. In fact, Lake Speed shows this in action in the video at about the 8:30 mark. When testing several diesel oils for cam wear, they found that the diesel oil with the lowest level of ZDDP showed the least wear on the cam lobes.

Furthermore, the higher detergents in diesel oil (about two to three times that of gasoline engine oil) interrupt the dispersal of the ZDDP. Many of the detergents in diesel engine oil are calcium-based, which creates an entirely different set of issues in direct-injected engines.

Is Direct Injection Engine Oil Different From Diesel Oil?

Diesel-specific oil is absolutely different than direct-injection-specific oil. In fact, DI engines need a slightly different formulation than port-injection engines. This is due to a phenomenon called LSPI — or Low Speed Pre-Ignition. Unlike typical pre-ignition, which occurs at the top of the piston, LSPI occurs between the piston rings, where there is never supposed to be any ignition at all. The cause of this is high levels of calcium in the oil. Using diesel oil in a DI gas engine will absolutely result in a catastrophic failure of the engine because diesel oils have 1,500-2,500 ppm of calcium. Current API-rated SP oil, which is designed for DI engines, contains less than 1,000 ppm of calcium.

This damage is the result of Low Speed Pre-Ignition — or LSPI. Unlike traditional preignition, LSPI detonates in the ring lands, resulting in the above failure.

What Oil Do I Need?

It is all based on the year and type of the engine. The American Petroleum Institute (API) has ratings for every motor oil, and they are coded to match the type of engine and year range. Gasoline oils are noted with an “S” prefix, stating it is a “spark ignition”, and diesel oils use a “C” prefix for “Compression ignition”. The letters or numbers after the prefix note the beginning year of the formulation. Current gas oil is SP, which is noted for 2020 and newer vehicles.

The short story is that you should use the correct oil for the engine you have. There are dual-purpose motor oils designed for both diesel and gasoline engines; however, these are compromised oils, they are not properly formulated for either engine. The correct solution is to use the correct year range C-rated motor oil in diesel engines, and the appropriate year-range S-rated oil for gasoline engines. When it comes to older gas engines, specifically those with flat-tappet lifters, you can use the current SP-rated oil with a ZDDP additive, or an engine oil that is formulated specifically for older engines without catalytic converters. Oils such as Lucas Hot Rod & Classic, AMSOIL Z-Rod, Driven Hot Rod, and others are designed for these older engines, which need more protection.