IIRC the S650G was sold in the US for somewhere around a decade. I believe it was from the late '60s to at least the late '70s. Unleaded gas was definitely around for a majority of that time. I've even owned a '72 Mercury that was unleaded only. I have 1 complete operators manual from a later year, part of an older manual, and a repair manual that is new enough to have quite a few variations listed. Both manuals simply list the fuel as gasoline and the newer manual does not appear to just be a reprint of the old. The repair manual was definitely not written in the late '60s and just re-printed. There is too much info in it about the various changes made to be a re-print.
Besides, the chances of you damaging anything is probably pretty slim unless you are using it everyday. I know I don't bother putting lead substitute in my '66 Mustang (I've had it for 15 yrs), my '74 F350 (I know the previous owner had it for at least 10 yrs and never added anything), or the '59 801(at least 10 yrs). They all run fine and I've had the engine apart on the Mustang and I know I specifically checked for valve seat damage and didn't find any. I also know lots of other guys with classic cars/trucks who don't, haven't had hardened seats put in, and drive them quite often and some of them pretty hard. Most of these guys built these cars/engines and know what they're doing. Most of the engines in these cars are worth more than an entire perfect S650 so I'm not going to sweat it.
