Quick answer
Most engine oil additives aren’t harmful if you pick the right formula and follow directions. They become harmful when mismatched to your oil or overused, which may cause sludge, reduced flow, or sensor fouling.
Detailed answer
Harm arises primarily from either ignoring compatibility—like using a high-ash additive in a DPF-equipped diesel—or thinking more is better, resulting in over-thickening the oil. Additionally, some older, metal-rich additives can harm catalytic converters or oxygen sensors in modern vehicles. Another scenario is using heavy seal conditioners on an engine that doesn’t actually have leaking gaskets, inadvertently causing overswelling. Following the product’s exact guidelines, verifying engine type, and carefully measuring dosage typically prevent these harmful effects, ensuring you only see the intended benefit.
