You could always run a uoa, I bet it would say 3 years would be fine though. I put two years on my hemi only 5k miles a year, still over 3 tbn left of course fuel injection. Like someone said, depends how it's running, you know those old muscle cars can be notorious for dumping fuel into the gas if not tuned perfect with plugs hot. So just off that possibilty and it being such a nice vehicle, I say once a year. I had a 69 chevelle white with the black top, had to sell it to a friend when I went into the Army. Glad there are still survivors.