20W is the yard stick by which all other "performance" oils are measured. Expand your search & get what the manufacturer wants you to use. I've used 20W motorcycle shock oil & 20W Yamaha brand motor oil & couldn't tell the difference. The point is that 20W allows the appropriate amount of vacum to occur at low rpm, yet allow the engine to accellerate smoothly to wide open throttle no matter what rpm you start at - just perfect for all around driveability. ATF, WD40, 3n1 oil are all better then nothing, but I also had a buddy that raced with SUs & he ran them empty because he only cared about high rpms & didn't want anything getting in the way of a wide open venturi.