NASCAR continued to use lead additives in its race gasoline until the 2007 Daytona 500, which led to concerns about the health of those exposed to the fumes of the cars (fans and residents living near the race tracks). Lead is a well-known environmental risk, but the performance needs of race engines (in particular, the high compression ratios) once made it difficult to switch to unleaded fuel.

In the US, the commercial use of leaded fuel has been phased out since the early 1970’s, when catalytic converters were required to be installed on new cars, making unleaded fuel a requirement (leaded fuel will destroy a catalytic converter). The sale of leaded fuel has been mostly banned in the US since 1996, but exemptions exist for auto racing, as well as aircraft, farm and marine equipment.

NASCAR eventually took steps to eliminate the need for leaded fuels. In 1998, NASCAR and then-fuel supplier Tosco (76 Product) conducted an unsuccessful test of unleaded fuel in selected Busch Series races. In July 2006, in the first in a four-week test run of unleaded fuel, the first race since 1998 to run unleaded gasoline, known as Sunoco 260 GT Plus, the same fuel used in road races, was held during a Busch race at the Gateway International Raceway in Madison, Illinois. The testing in July 2006 was successful with no suspected engine failures or malfunctions from the new fuel. In October 2006, NASCAR stated its intention to transition to unleaded fuel in all three top series (Craftsman, Busch and Nextel Cup) in 2007, with the exception of the Daytona 500.

The first race of 2007 in which unleaded fuel was used was the Auto Club 500 at California Speedway on February 25, 2007. There were a number of engine failures during the race, leading many to believe that the unleaded fuel is to be blamed. The drivers who encountered failures include Dale Earnhardt, Jr. and Martin Truex Jr. of Dale Earnhardt, Inc. and Kasey Kahne of Evernham Motorsports. The engine failures of both Earnhardt Jr. and Truex Jr. were attributed in part due to the lack of a lead additive, but also due to centrifugal force causing improper distribution of oil between the left and right sides of the engine. Evernham Motorsports has not disclosed the reason behind Kahne’s engine failure.