[Johnny]

Can anyone please advise me on what fuel to use for my 67 289 Fastback? I would like to know if I need to use an additive with unleaded fuel. Do I use 95 or 98 octane? I don't know if my car has hard valve seats or not so any advice would be greatly appreciated thank you.

[Neko-]

To my knowledge (I don't drive an older Stang) you do need the additive... As to using 95 or 98, I'd start with 95. IIRC the fuel in the US is 81 octane nowadays. If it runs rough with the 95 (or select the cheapest, since 95 is the cheapest here), try the 98.

Since I'm not THAT big on the technical matters, it may be wise to throw this question into the http://www.allfordmustangs.com/forums/classic-mustang/ forum. That should provide you with some insight on how to proceed.