The 300 seems to be very susceptible to carbon build up on the tops of the pistons . It doesnt seem to take much to cause spark knock problems using using lower octane fuels . What I occasionally do is run some Marvel Mystery Oil Fuel treatment & that seems to cure the knocking problem & I can use 89 octane with no trouble . Run the tank down to 1/4 full , pour in the fuel treatment before you fill the tank & drive it . It may take a 2nd treatment , but you will usually notice a difference after burning the treated fuel .