Quick answer
Oil additives can be harmful if they’re incompatible with your oil or used improperly. The wrong formula or overdose might cause foaming, sludge, or disrupt oil flow, but correct use is generally safe.
Detailed answer
Some drivers fear that aftermarket additives will degrade seals or clog passages. Indeed, choosing a random product—especially with metals not suited for your emission system—or overusing seal conditioners can create problems. Excess metal content might foul oxygen sensors or catalysts. Over-thickening the oil can hamper flow on cold starts. However, a well-chosen additive designed for your engine type and used as directed typically doesn’t pose a risk. As with any tool, it’s about matching the right product to the right need and following instructions. That’s how you avoid trouble and see benefits instead.
