| This is a question I raised after a recent head gasket job on my beloved '85 318i.
Down here in Australia, I'd slavishly been using "Lead Replacement Petrol" for over 12 years. This is the stuff the gas companies marketed to be used by pre-1986 domestic and Japanese cars that mostly ran on the old "super" fuel (ie: leaded) which the governent banned. It also cost a couple of cents more more than unleaded but far less than premium unleaded.
LRP is now in the throes of being phased out and within two years probably won't be around. Based on what I now know, I see this as is a good thing.
It turns out I should have been using "unleaded" all along. My mechanics have all recommended I switch to (what we call) premium unleaded (eg: Shell Optimax). They all said that LRP left a fine brown residue everywhere and was basically a bad idea imposed on an unwitting public. Apparently BMW's of my vintage were engineered to not use leaded petrol (like many Japanese cars where I guess leaded petrol was outlawed early?) yet locally built GM. Mitsubishi and Ford were still being designed to run on leaded fuel. It took until 1986 to catch on.
Reassembled, the car ran really badly (after the head repair) which was tracked down to blocked injectors. (same crud?)
So the official line I got is to only use premium unleaded and nothing else - it's currently about AUD$1.13 litre at the pump (about US$0.83)....which after $2,500 in repairs, I'm "happy" to pay the extra few cents if it keeps Heinz healthy. | 
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