What type of gas is best for a 1999 Mercury Cougar?

Answer 1

The 2.5 L Duratec or the 2.0 L Zetec power plants available in Cougars from this generation could, and should, run regular 87 octane gasoline all day.

Running higher octane does no good for a car that does not need it. The only reason to use high octane would be if you had had an engine out of a Contour SVT put in your Cougar. The reason for this is that octane is a rating scale that describes the tendency for a fuel to resist auto-combustion without a spark, otherwise known as dieseling or knocking. This tendency decreases as your compression ratio increases; therefore, the higher your compression ratio, the more likely you are to need mid grade or premium fuel.

As for brand and or additives, no conclusive evidence has ever been found that "fuel additives" perform as claimed, and most of the gas is made from the same crude stock, some stations, even though they are different brands, get their gas from the same company too.