Quick answer
Most additives mix best with warm oil, helping them flow and blend thoroughly. However, always check the label; some say add when cold. Typically, a slightly warm engine ensures better distribution.
Detailed answer
When oil is warm, it’s less viscous, so the additive can disperse quickly throughout the crankcase. You don’t want the engine scalding hot, though—just warm enough to ensure good flow. Idle the car a few minutes, shut it off, then pour in the additive. This approach helps the product mix well without risking burns or friction from super-hot engine metal. If the label explicitly instructs otherwise, stick to those instructions.
