Octane address's the concern of Detonation - "pinging" under acceleration....there is actually little to no chance of your engine 'pinging' on any pump gas 89 or better. The concern should be cleaners....you should use a fuel containing cleaners to keep the injectors and valves clean. It is also reccomended to change brands every several fill -ups to prevent buildups of carbon deposits internally. Studies have shown the engines that have used one brand of gas exclusively have higher levels of build up than engines that use a variety of brands.
Unless you have problems with high tess pump gas you do not need to pay for higher octane fuel. The higher octane allows you to run higher compression, turbos, a sc, or in a carb car it will retard knock, without having to change timing.
The Jalpa and Countach are designed for 98-100 Octane (European Spec.) whereas the Diablo runs with 95 Octane.
The higher Octane rating means a lower self combustion, therefore the lower knocking ability, but this means also it needs more ignition power to start combustion at all. If your ignition system is weak it might possibly have problem to ignate a high Octane fuel mixture at the correct point.
I have to admit I have no idea at which octane rate this might occur... I think a bad mixture (lean or rich) has a higher influence.
If you use 98 Octane Euro Spec. you are on the safe side with the Jalpa and Countach, I also used 95 without any problems. My MSD-Ignition burns almost every mixture
The reason I only put the 100 octane fuel is because I put 91 octane (which is the highest in CA) at a Chevron next to Ferrari of Beverly Hills, 10 minutes later I got a check light. Come to find out it clogged the injectors and I had to get them cleaned.
2 weeks later I see the gas station shut down for renovation for the tanks.
Since that experience I would rather be safe then sorry so I just put the 100 octane gas because you never know with these cheap gas stations.
Mercedes-Benz recomended Chevron with Techlon (not sure of spelling) to clean my injectors on a S55 Supercharged AMG engine and it worked. I would recommend Chevron with that adative which is in their middle and hightest gas. In fact, MB told me that I could use the middle brand (91 oct?) because it had more Techlon in it. One to Two tankfulls is probably all you will need.