Hi Malc.
For what it's worth, here's how the two previous owners of my 230 dealt with the question. The first (a golf nut) kept five golf tees in the ashtray. Each time he filled up, he did so on unleaded, and then moved one golf tee from one side of the ashtray to the other. When he got to the last tee he put in a tank of LRP, then started over.
The second owner kept a bottle of lead additive in the boot (trunk), and each time he filled up on unleaded, he added a capful of the additive.
For mine, the engine 'pinged' on the lower octane fuels (perhaps curiously?). My mechanic took the same view as mentioned here by one or two others (see Ben below) - you won't get better modern valve seats, and you don't need them. I took that as a licence to run on the highest grade unleaded available, and couldn't be happier. As for the environment: I'll have to make up by recycling my garbage and composting!
Cheers,
kns.
1965 230SL, Manual
[1965 220SEb Coupe, Man.]