I have a 70 eldo w/ the original 500 and have been using 93 octane, is this the best choice?
It will most likely run best on 93. I have a '75 Eldorado with a 500 and if I put anything lower than that in the tank, the valves start to rattle. If money is really tight, use the mid grade. You'll only get a slight rattle from the valves.
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