You don't really need a lead additive with the original valve seats unless you're operating under high load situations a lot (ie. towing, climbing hills, or WOT). I've talked to a couple people who've taken original Mustang 289's apart after running standard unleaded gas for 20 years and 150k miles without any valve recession at all.

As for octane, you want to run the minimum octane possible without detonation (pinging). You want to make sure the timing is set properly before you go and figure out what grade of gas to use.

I'd recommend staying away from bargain gasoline. Costco gas is crap, Arco isn't a lot better. Everyone I know that has temporarily switched to cheap gas from Chevron, 76, Shell, etc has noticed a significant drop in mileage (~15%) for the cheap stuff. I try to run nothing but Chevron 89 octane in my 331. It's about the same, if not cheaper than 76 and Shell around here, and it's got Techron in it

 

Nothing but 92 octane or higher for me, otherwise it knocks like a bitch under WOT
i sometimes use a lead additive in mine if i have the money, -> original engine not ever rebuilt to my knowledge.

 

i use 87 haha and my mother had a 99 gt that she nvr put anything but that and it nvr had trouble running but the higher burns a lot cleaner mayb if gas was a dollar a gallon again id get premium[8D]

 

Chevron premium for me because I can't afford not to I think I get 7-8 miles per gallon around town. Sometimes 2 or 3 if you know what I mean.
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eh, i guess i use the lead additivedue to the car only having 1000 miles put on it in the last 10 years... 28,800 in the last 43 years.

 

+1

Chevron is not available here, so I use Mobil Super.

Bargain gas most assuredly IS crap, they leave out the additives that lessen valve deposits and that's why they are cheaper.

 

Not only the additives, but the gas itself is just crappy gas. My car actually runs worse on Costco gas than on high end gas. If it was only the additives, mileage wouldn't suffer. I've heard (but can't prove) that many of these cheaper companies buy gas only from the bottom of the tanker.

 

I have hardened valve seats and run 87 octane on my 66 with zero problems. I purchased the car from a Chevron employee who formulates their gas and he said the 87 octane was all I needed.

 

Currently I use supreme but, as soon as it is available I am planning on going to an ethanol setup... My engine is a high performance motor though so to me running ethanol makes since.