Short answer : Just keep running 98 octane fuel on its own.
Long answer:
Now reasons for running correct octane fuel...
When you run a low or insufficient octane fuel, your engine pinks (detonates, pinking, pings, knock what ever you get the point.) Under load.
Now this causes huge increases of heat in the combustion chamber. Now what this does is causes your exhaust valves to get to point where they begin to melt on the surface. Now generally under high load and/or high-RPM this can/will cause engines to fail.
Now you don’t _have_ to run lead or an upper cylinder lubricant. Under normal every day driving conditions your exhaust valves wont get hot enough (using fuel of sufficient octane) to cause valve seat recession. Even under higher load scenarios this will mostly be the case as well.
I Oz we have had fuels with 98 octane like Shell Optimax and the mobil, BP & Catel Vortex 98 these generally exceed the octane rating for most cars. As you have sufficient octane the car wont pink, because it isn’t pinking you have lower combustion temps so your valves don’t get as hot. As they are not getting, as the valves don’t micro/localise weld themselves to the seats so you don’t get valve seat recession.
note Regular unleaded is around 94 from memory & premium is 96 along with caltex vortex which basically is premium with injector cleaner. Optimax and the other flavours by mobil & BP are pretty much the only true 98 octane fuels.
If you drive an older car like a VB-VK and it is driven in a low-stress application, basically used as a daily driver, then you need have no real need to run upper cylinder lubes or lead. As long as you use a fuel that meets or exceeds the octane rating for your engine. Really the only time you hay require an upper cyl lube is high sustained loads and RPM all the time.
If you rebuild your engine the best way to overcome this from ever happening is to install hardened exhaust valve seat inserts and exhaust valves of harder materials, such as 4N stainless.
Also if you lower your compression this will enable you to run a lower octane fuel. Simply retarding your timing isn’t really the proper way to tune an engine to run a lower octane fuel. Although you can advance your timing when you run a higher octane fuel this is because the cars timing was retarded from factory to pass emissions laws etc.
If your car pinks for a longer period of time you may experience pre-ignition or overrun when you turn your engine off. Basically what’s happening is your cylinder is so hot it’s igniting the fuel without the need for spark. This generally wont last long as your engine will destroy itself pretty quick.
The early commodore engine were designed for 97 octane super. If you run 98 octane fuel you wont need to run an upper cylinder lube. Super used to contain approx .14grams per litre of lead, Potassium, Manganese, Sodium & Phosphorus based upper cylinder lubes wont do the same job as leaded fuels, they burn at a lower temperature than lead does. Although they may aid in protecting they wont do the job as well. Sodium based substitues have been linked to damage to turbo charges.
Most additives will also contain a octane booster component. The higher the octane fuel the cooler the compustion temperature will be.
Cheers
Scott
