unleaded vs. leaded for '67 straight 6

[Lefty&Son]

Hi Guys,

First, thanks to Rod for answering my previous VIN question. Not sure about this board...others I've used don't want to "waste" posts with thank yous...anyway...THANKS!

Second - I've got 2 differing opinions of gassing up my "new" 1967 GMC - NO WAY can you put in unleaded, and You can if you're not going on long trips. The second guy mentioned some additive (Marvel Oil?) that I could add.

I am soooo new to this, scared to damage anything. What do you all think? Any opinions would be greatly appreciated.

Thanks,
Lefty