There are all kinds of products on the market , some work some don't work!
There is a lot to be said for just what is hi mileage. Sometimes if the engine is performing well , even with hi mileage, changing it's diet can sometimes cause more problems than it solves. If good maintenance was done regularly, you could maybe change to a better grade of motor oil, like synthetic, and possibly receive some addl benefits. Improved quality plugs also can produce some benefits. If the vehicle was used locally and not run on the highway much, sometimes a tank or two of premium gasoline will do what canned products do.