What cars can be used for a drivers test?

Since the early days of the Busch Series, many Cup drivers have used their days off to drive in the Busch Series. This can be for any number of reasons, most prominent or often claimed is to gain more "seat time", or to familiarize themselves with the track. Examples of this would be the first ever winner of a Busch Series race, Dale Earnhardt, and the winner of the most races in Busch Series history, Mark Martin. In recent years, this practice has been termed "Buschwhacking" by those that criticize the practice, claiming that Cup drivers racing in the Busch Series takes away opportunities from the Busch Series regulars, drivers that are usually younger and less experienced. Proponents of this practice, however, claim that without the Cup "superstars" and the large amount of fan interest they attract, the series would cease to exist.