Recently began adding a cocktail of PowerService and Marvel Mystery Oil to the tank when I fill up. ('85 300sd)

For the past year, I have added a dose of PowerService to every fillup. Engine runs much quieter/smoother, and hills are a little easier to climb.

But, I was surprised how much smoother and quieter the engine runs when I add about 8oz of Marvel Myster Oil to the fuel. Quite the difference.

However, I am wondering about the long-term effect of using these products at every fillup. Is any short-term gain cancled by the long-term effects of adding these chemicals?