Congrats on the purchase and the fact you can get 99 Research Octane fuel for it!
There are other additives in the fuels that can do the same thing as Tetraethyllead did in prior times. From an article in a motor home magazine, from the earlier '80s, there are two types of lead-substitute additives. Sodium based and lead based, I believe. Being that lead is a deadly poison that can penetrate the skin, even the ones which used to be sold in the USA were VERY "watered-down" such that a full quart would not put the fuel up to even "low-lead" standards.
The fuel additive I used to buy for my '67 Newport took about 2 oz per tank, so I presume it was sodium based? But that came to a halt one day when the accel pump on the ThermoQuad I had on it stopped working. When I removed the primary venturi cluster I discovered that the anti-pullover weight under the cluster, from the accel pump circuit, was firmly stuck in its bore. I got it freed-up and removed for inspection. Some sort of clear residue was around it. I cleaned it off and reinstalled. No problems past that. Also didn't use any more sodium-based additives, either.
I'd recommend checking with your regional Shell gasoline people and see what their recommendations might be, for older vehicles which were designed to run on leaded fuels. There could well be something available over there that we don't have available in the USA.
Before Chrysler went to induction-hardened exhaust seats, like for the '72 model year, they recommended one tank of three be leaded fuel, if the other two were unleaded.
I also found a paper in a Chilton magazine that Chrysler did. They took a low-lead spec '72 Town & Country wagon, with the HD trailer package and 440 V-8. Hooked it up to a trailer that put the combined vehicle weight at the max spec they recommended for that car. Then they put it on the test track for a durability test on straight unleaded fuels, with the non-hardened valve seats.
By 12K miles, the heads were trashed from valve seat recession/erosion. That's under heavy load all the time. Much more than what a normal street car would see, typically. In other words, something that's not going to happen "over-night" or in one tank of fuel . . . unless it was already in such a state that it was getting ready to happen anyway . . . any you got to be the lucky owner when it did.
In the mean time, drive and enjoy the car for what it is. Check the age of the tires, too! Anything close to 6 years old needs to be changed before it comes apart/fails. Look at the condition of the rubber fuel lines (fuel tank forward) for deterioration from ethanol'd fuel blends. Might plan on a new fuel pump too, for the same reason, even if it's not leaking now.
Enjoy!
CBODY67