thanks for the replies! I guess from now on I will go with cheapest gas and will decide myself if I want to spend saved money on any additives ...

in the country I spent most of my life, the lowest grade gasoline (nominally 76) was mostly the same everywhere, but higher grade gasoline (nominally 91 and 95) differred significantly from station to station depending on how much of "additive package" (mostly lower grade gasoline ) has been added to it...