stock slant six , which grade of gas should i use ?

One thing about that I would like to mention is that I run 87 in my 318 most of the time, but if I run 91 and bring the timing up to accommodate it, I get more miles out of the 91 than with the 87.
So the 91 cost more and I get less gas for the money, but I get more miles to the gallon, so it's a balancing act.
You do have to watch the prices and realize when that gets too far out of balance prices wise.