I seen a video from Chrysler dated 1973 saying to use lead substitute every 4th tank of gas. I've used MMO every other tank in my 66 Dodge Polara 383 for the past 2 years so far with no issues. The commercial lead subs are a joke/scam to me. I bought a case of Gunk lead substitute and it is repackaged kerosene at a crazy price. MMO, trans fluid, kerosene, 2-cycle oil will do the job IMO.