I started a thread posing this question some time back.
I have a 1983 Euro 280SE which MB recommended an unleaded conversion when I posed this question to them. I had been placing lead substitutes regularly, but was planning a head job due to leaking valve seals with heavy oil consumption and plug soilage.
After cleaning the parts, the valve seats were still in a pristine condition with no sign of wear (after 20years/180,000km). Talking to the machine shop, who have handled a few w123s conversion, all that needed to be done was to replace the valve seats. However, they have not seen any valve seat problems all this while with cars who have run unleaded against MB recommendation.
A poll of independant MB workshops in my area have unanimously concluded that valve seat recession is a myth and unleaded is safe to use. I would agree with that: I have not been able to obtain hard evidence, and have found a lot of "hearsay" putforth.
However, I still put in lead substitutes occasionally out of force of habit and as a hedge just in case......