I've heard conflicting stories regarding the necessity of harden valve seats and or fuel additives. I'm running with the theory of convert to harden valve seats if using engine in heavy load type driving such as mountain climbing or lots of acceleration. Otherwise, let it go until the engine or the heads need rebuild then convert.
I'd be interested to hear what our engine gurus say.
If you're engine is fine now, it could be for several tens of thousands of miles, so there is no immediate worry about valve seat recession (VSR).
You can prolong engine life on unleaded fuel by not revving above 3,000 rpm frequently (no problem for street driven V8s) and avoiding high loads.
If you have solid valve lifters which require resetting every 10,000 km/6,000 miles, you can keep an eye on the progression of VSR. If the exhaust valves' clearance close up, you know VSR is progressing. Generally you have ample adjustment space to compensate for VSR for many many miles. Only when this has been used up is it time to have hardened exhaust valve seats (inlet valves don't suffer VSR) fitted. On hydraulic lifters it's harder to see VSR progressing, and frankly, I don't know how.
If you want to postpone rebuilding the heads as long as possible, e.g. until the whole engine needs a rebuild, you can use a lead replacement additive. The Federation of British Historic Vehicles Clubs (FBHVC) has had tested many lead replacement additives. For more information about VSR and the FBHVC-endorsed additives take a look at http://www.secant.co.uk/unleaded/fbhvc-press.htm
As mentioned in a previous post, the standard 87, 89, and 92 octane. But Shell, Mobil, and Sunoco also offer 93 (which I use in my Audi) and Sunoco also offers 94. I think a few of the others offer 93, but I've had trouble with it so I stick to the majors.
As long as you don't regrind the seats the so called "curing" should stay in effect for awhile. However, without the constant flow of lead over the seat to replenish what was originally impregnated into the seat, the seat will pound in over time. How much time depends on you and how you drive it. I strongly recommend having the hardened exhuast seats installed for longevity and peace of mind.