Back in 1968, I worked in the Valvoline garage at Indy, mixing fuel and dispensing oil to those that were sponsored by Valvoline. Many of the drivers wanted their fuel spiked with nitromethane for qualifying since they only had to run 4 laps to qualify. Dan Gurney was running 2 Ford engines at that time, one OHC and one pushrod and qualified both. On race day he asked me to mix 15% nitro for the pushrod engine but demanded that I tell no one. Perfectly legal, but no one could expect to run nitro for 500 miles and keep an engine together. If I remember correctly, his cars finished 2 (OHC) and 3 (pushrod) and to this day I have no idea what his builder did to that pushrod engine to not only keep it together but finish 3rd on 15% nitro.