Like mentioned before...If you work your engine hard, you probably want to run a lead substitute.
IMO Here is what I would/wouldn't do.
I would run unleaded without additives. This works for me because I use a newer vehicle for working, and I use my Stovebolts for pleasure.
I wouldn't run out and do a hardened seat valve job just because I wanted to work the old Stovebolt. I would run it until I needed a valve job, and THEN consider the hardened seat valve job.
With regular driving (not hard work), I would expect a 10-25% decrease in the life of the original seats using unleaded gas. That's 10K-25K miles in 100K miles.
Considering the costs of a hardened seat valve job, I wouldn't bother with hardened seat replacement (unless I'm already having a valve job). I would use a lead substitute until a valve job was needed "IF" I were overly concerned about my valve/valve seat life, and I didn't mind the hassle of adding to the gas, or the price of the lead substitute.
Bottom line? Run lead substitute......
1. if it makes you feel better.
2. if you plan on "working" the truck.
3. if the cost of hardened seats and a valve job isn't in the cards for you right now.
4. if you question whether or not you need a lead substitute.
Otherwise, if you can live with shorter valve life, and replace the seats when a valve job is needed, run unleaded.
Just do what makes you feel more comfortable.
