Th theory is that anything with an iron head doesn’t have hardened valve seats and needs a lead replacement additive.
The lead in leaded fuel provided a “cushion” to stop the valves recessing into the valve seats.
That’s the theory, but it is increasingly becoming clear that most iron heads are lasting just as well with unleaded fuel and no lead replacement additives.
I would go a step further and say that Volvo used a much better quality of cast iron than Ford or Holden did, so that’s probably worth considering too.
I would also say that in 2024, most cars with iron heads aren’t being used every day, and probably get a lot more maintenance per kilometre driven than they did when they were new, so that’s worth considering too.
As to fuel… use whatever stops it pinging.
If you are not aware, pinging/detonation is the fuel combusting by itself, before the spark plug has fired. It has the potential to destroy your engine, so should be avoided.
It sounds like someone hitting the engine with a hammer, or a few lumps of gravel being bounced around inside a Milo tin.
Usually occurs when the engine is under a lot of load, like accelerating away from a stop, uphill.
I would expect you’ll need at least 95 octane fuel to avoid detonation in your engine, but it might need 98 if it has had the head machined a few times.
DON’T use ethanol fuel in this car - not because any of the old wives tales about how it will eat your face off and turn your family into toads, but because it turns acidic if it is left to sit around.
If the car is used regularly, then E10 is NOT a problem, but it does cause dramas if it gets old.
If your car has Stromberg carburettors, then all modern fuels will eat the diaphragms relatively quickly. I don’t know if anyone is making them to survive modern fuels yet.
