Hi Zee,
Like most everyone else, I run the highest octane available and keep the valves adjusted regularly. One of my motors has hardened seats and I do not know about the other, but have not had any problems.

One thing I would want to find out is what kind of carbs and intake manifold it has.
'69 Volvo's were fitted with Stromberg carbs and, what I believe George refers to as, "the pipefitters nightmare" intake manifold. The manifold can be identified by extra "butterfly" flaps in the cast iron runners. If you accuate the throttle to wide open, you'll notice about halfway through, an arm on the carb accuates a little rod on the manifold.
If there is any slop on the extra butterfly rods, you will have a lean condition that could cause damage to the pistons and cylinder walls.
Could be someone has changed it to the early aluminum intake manifold and changed carb style by now but, from past experience, I would want to know.

Carbs can be identified by very squatty domes. Of course if you've nothing to compare...maybe go to google and do an "image" search for SU carbs and then look up Strombergs. Chances are you have a black plastic airbox?

Best,
Shayne.