"Gum & Varnish" residue is a bi-product of gasoline itself.
Take a glass, fill it about 1/2 full and let it stand
outside until all the gas evaporates. Any brand, any grade will leave a reddish
or black gum/varnish residue.
This is what is screwing up all the injectors of the
world. It upsets the spray pattern of the injector, sometimes clogging or
completely clogging the injectors -- doesn't do much for carburetors
either!