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I add it in at the 250,000 mile mark. So far, I've not kept a car that long so I've never had the need.
As for synthetic, it is better than dino oil, but unless you have a high performance car and drive at high rpm and high temperatures, it is not going to get you much, if anything. Most engines today outlast the rest of the car with oil changes done according to the recommendations. I change at
7500 miles and can get 200,000 miles from an engine. The exception was my
83 Olds V-6 that had poor design problems.
If it makes you feel good, spend the money. IMO, it you need additives, you really have other problems.