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"...
Bad for your engine, you ask? Probably not - but are they actually all that good? Many of them aren’t, while many of them mischaracterize the benefits they can bring. They'll make claims about doing certain things (like "stabilize the oil") that the oil doesn't actually need help with.
Indeed, the more you survey the market of “oil additives”, the more you get the distinct impression that a large chunk of that market relies on a certain kind of “black box” characteristic for engine oil. We know it’s in there and we know it’s important. But what does the average person know about what oil is supposed to do? How can they tell if the oil has been improved by adding an additive that they bought?
Whether an additive is having an effect is easier to tell with fuel, where we can always point to gas mileage or some perceived improvement in power and driveability as a way to show that a fuel additive is working. Oil doesn’t work that way - there aren't any discernible signs to the driver that the oil is affecting the vehicle’s performance in any way, good or bad - or that an oil additive you added is doing anything. You just have to have confidence in the choice you made, for better or for worse.
Are oil additives bad for your engine? Not really, no. People think that they need an oil additive to “improve the lubrication” for their oil. But they don’t really need that because today’s high-performing oils do that job plenty well. They don’t actually need much help there.
What they can use is help with their other functions - keeping the engine surfaces clean and free from particulates, neutralizing acids, etc. There are some good oil additives out there that extend the oil’s ability to do those important jobs. Those do have real value.
Remember, though, that if you use one of these oil additives, you shouldn’t interpret that as permission to deviate from the recommendations of your engine manufacturer for when you should change the oil. We always recommend that even if people use an oil additive like X-tra Lube, they should still do what their engine manufacturer recommends. So in one sense, an oil additive could be bad for your engine if you let it give you a false sense of security on changing your oil.
Are there any bad oil additives? Not in the sense that they’ll damage your engine (though we can’t speak for the entire additive universe, since oil additives aren’t regulated in the same sense that fuel additives are regulated). Oil additives based on ceramic or other particulate bases might have an increased chance of plugging the oil filter, especially with today’s higher-performing filters that filter out smaller and smaller particles. Most of the other non-particulate treatments, like the “viscosity modifiers”, they really don’t do anything one way or the other. They’re not bad for your engine, but they’re also not good for your wallet.
So let's get to the bottom line answer. The great majority of oil additives aren’t bad for your engine. But since they don’t really do a whole lot, and because you can’t really tell if they’re doing anything, they’re just bad for your wallet.