Dexron III and Mercon V are similar types of automatic transmission fluid, with the main difference being that Dexron is the formula developed by General Motors, while Mercon was created by Ford. The specifications of the two formulas are so similar that the two are often considered interchangeable, even if the car manufacturer recommends only using its specific formula.
Due to the similarities between Dexron III and Mercon V, many oil companies also produce multivehicle fluids that they claim meet both specifications and can be used in a number of different makes, including GM, Ford, Chrysler and numerous import brands. While this may be true in certain cases, the type of transmission fluid a vehicle uses depends on the make, model and year. For this reason, it's important to check the vehicle's owner's manual to ensure the correct fluid is used, or otherwise the transmission could be damaged.
Dexron III was released in 1993 and was designed to be compatible with all older GM models. It continued to be GM's standard specification for transmission fluid, but it was replaced by Dexron VI in 2006 and is no longer produced.
Mercon V was developed in the 1990s and was one of Ford's most commonly used transmission fluids until it was phased out in 2008 in favor of Mercon LV. However, Mercon LV and Mercon V are not compatible, so any Ford vehicle manufactured before 2008 requires Mercon V.