How many different kinds of engine oil additives are there? Do they really work? To answer that question, we first have to separate "finished oil" additives from "aftermarket" additives you buy on the store shelf.
Viscosity modifiers help the oil function in both hot and cold environments, essentially modifying the oil’s weight (for lack of a better way of putting it) to make it effective at both ends of the temperature spectrum. Detergents help clean the surfaces that the lubricating oil touches, keeping them free from deposit buildup. Dispersants function in a similar manner, picking off particulates from those same surfaces, drawing them into the oil film and keeping them away from causing friction and damage. Acid neutralizers do what the name implies, which goes a long way towards reducing the potential for damaging corrosion. All three of these are essential components to any finished oil.
On the other hand, this is where we have to move from finished oil additives to “aftermarket additives” – oil additives you buy off the shelf and add to the finished oil that’s already in your vehicle. They’re the bottles you see on the shelf at your local auto parts store, Autozone, O’Reilly's or whatever. Buying and using one of them adds extra cost to your oil change, so you want to make sure you’re not throwing your money away. Are there any of these oil additives that work?
As with many kinds of questions, if you ask “Do oil additives work?”, the best answer is “it depends”. More specifically, it depends on what they’re claiming to do. We haven’t tested every single oil additive out there (that would be a boatload of testing), but what we’re really looking at is: “Is an oil additive likely to be able to do what it claims it can do?”
So let’s talk at some common claims for aftermarket oil additives, without naming any specific brands (so as to protect the innocent or not-so-innocent), and look specifically at the claims they make. You can tell a lot about whether oil additives work by what they claim they will do for you.
This is the one that piques everyone’s attention. Who doesn’t want to get better mileage, right? The theory is that an oil additive can make the engine more efficient, and so it gets better mileage. By what mechanism, exactly? The most common one is through reduced friction in the engine. On one level, it kind of makes sense. If the engine components are turning more easily, the engine doesn’t have to work as hard to generate the “work” (i.e. distance traveled) per unit of fuel. Better mileage is the result.
But let’s take a step back and reconsider this. For one, there’s a lack of scientific and testing evidence that any kind of oil additive can substantially improve gas mileage. That certainly was the case with the Teflon-based oil additives that were popular in the late 1980s and 1990s. Teflon was the “slipperiest substance on earth”, and so it would improve your gas mileage by reducing friction in your engine. It turned out that not even Dupont wanted to support this claim (and they tried to stop selling Teflon to additive blenders as a result, but lost a court case and were forced back to selling it).
The truth is, any change in engine performance related to reducing friction in the engine is going to be so insignificant, it probably can’t be detected. That means if you come across an oil additive that is so bold as to promise you mileage improvements of 10%, 20% or more, you can safely assume they do not work and aren’t worth the money.
This one, too, might catch the eye of the car enthusiast. Truth is, however, this kind of claim is fraught with danger and should be regarded with extreme caution.
Let’s talk about the theory basis first. Why do you have to change your oil? It’s not because the oil itself breaks down, but more because the additive components in the oil get used up. Without them, the oil isn’t able to protect like it needs to. If you can supplement these additives by adding to them through the addition of an aftermarket product, in theory, you could extend the oil change life.
Here, the theory runs into the reality of liability. Every engine manufacturer has a recommendation for how often you should change the oil. Those aren’t arbitrary, random numbers they generated. They made the engine and they know best how to keep it running well. You should always follow the manufacturer's recommendation on not only how often to change the oil but also the kind of oil to use. They are recommendations for a reason.
This means if you come across an oil additive that promises that you can extend your oil change intervals beyond what the manufacturer recommends, you probably shouldn’t take that chance.
These additives are a little more specific than the ones making the broad claims above. Oil stability can be summarized as the oil’s ability to resist change over time. But what kind of change are we talking about? They don’t mean reduction in the oil’s additive content; yet, that’s the biggest change that happens to an oil over its working life. The change they’re talking about is changes to its consistency and makeup of hydrocarbons. They want to make sure the oil resists the formation of polymers that could be characterized as oil sludge.
This kind of benefit is more specific than a claimed benefit of “better mileage”. And oil stabilizers do actually work, if you keep your focus on that specific thing. The question that comes out of this is, how much of a benefit is that for the end user? Stable oil is better than sludgey oil. But is that really such a big deal? We would argue that the lubricating oil uses up its additive content long before you have to worry about the base oil becoming unstable. That would mean that the benefit claims about oil stability might be a little overblown.
Not only that, but competitiveness in the marketplace has led some oil stabilizer manufacturers to tack on to their benefit claims and imply, whether directly or indirectly, that using their oil stabilizer product gives you all sorts of other benefits like better gas mileage or more power. There’s no evidence that an oil stabilizer can meaningfully affect either of those.
There are other benefits we could cycle through, but that would make this blog post more like a blog book. The bottom line? Stay away from oil additives that seem to promise the world. Good oil additives know their limitations and they know their chemistries. They don’t have to rely on pie-in-the-sky promises to trick or confuse consumers. This means stay away from big claims on mileage improvement and power increase. Instead, look for oil additives that supplements the established chemistries already present in the finished oil – better detergency, more dispersants for cleaning, more acid neutralizers for protection.