Unleaded and valve seats etc.
In Australia, unleaded fuel was only introduced in 1986. From that time onwards all new vehicles were fitted with catalytic convertors and were required to operate on unleaded fuel. From 1986 until only very recently, leaded fuel was still available for use in older vehicles, although its lead content was progressively reduced (which also resulted in the RON dropping from a previous 97 back to about 94). Currently, a lead replacement fuel is still available for older vehicles or you can use unleaded and add your own lead substitute.
At the time of the introduction of unleaded, Mercedes-Benz Australia published a list of their vehicles that were suitable for operation on unleaded. It clearly stated that all pre 1977 models were not suitable and required leaded fuel. From 1977 until 1986 some models were still unsuitable, some were suitable with every fourth tankful being leaded and some were suited for permanent use with unleaded. Where leaded was required, it was to preserve valves and valve seats and in the case of some higher compression models also due to the inadequate 92 RON initialy available with unleaded. They also stated that this list applied to Australian delivered cars and that privately imported vehicles may differ, despite us usually getting Euro spec engines.
I currently run my '73 350SLC (M116 high compression) on 98 RON unleaded with Valvemaster lead substitute that I add myself. In this way I am getting a fuel of required octane rating (owners manual states minimum 97 RON) and also avoiding differing lead substitutes of different brands of lead replacement fuel (which is also only 95 RON).
Until such time as someone can prove to me beyond any doubt that my SLC can run without lead or a substitute, I will adhere to the advice given by Mercedes-Benz Australia at that time.
107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.