Valve stem and spark plug deposits are certainly caused by lead, as is probably a lot of other crap one finds in there. However, 0.14g/l TEL is a “lot more lead” than unleaded petrol. Hard to say of course but it probably helps quite a bit. So I don’t think the historical fact that the old engines were originally certified with “MOGAS” is very helpful.
Today’s new “MOGAS” certification will be a different thing, because it will be lead-free. The issue is the ethanol and fuel system materials…
Either way, there is significant “track record” with mogas (TODAY’s mogas) use in Europe. It does work, leads to overall less maintenance, is cheaper per litre, etc.
Just to explain why people in especially NL, GE, AT are interested in this and that they aren’t stupid.
I’m not aware of any aircraft engines being certified originally in the 1970s or before to use anything but 80/87 (or higher grade) Avgas. Most 80/87 was unleaded, or so I understand, although the 80/87 spec allowed a little lead so that 80/87 could be carried in the same tanks as 100/130 etc.
FAA STCs for auto fuel that were available starting in the 1980s were for unleaded fuel, simply because leaded auto fuel wasn’t sold by then in the US market. It was unavailable from the early 80s, due to US car emissions regulations requiring catalytic converters that are fouled by leaded fuel. My O-320 and many other engines like it were run on premium unleaded auto fuel from the 80s until MTBE contaminated groundwater became an issue circa 2000, MTBE was mostly phased out, and alcohol was made the octane enhancer of choice for US auto fuel. That made the STCs inapplicable, by that time mostly-unleaded 80/87 had also been phased out, and so for the last 20 years or so leaded 100LL has been the only available fuel for aircraft engines.
By the way, this is what Vliegwerk Holland says about TB9/10:
Yes, this O-320-E story goes round and round 
However, if you just want to do an STC for E0 then fuel system material compatibility is not an issue, and an engine-only STC should be “technically safe”.
If you want to do an STC for E5/E10 then fuel system material compatibility is an issue and you need to start with Socata, and without their help the project would be dead. They can change their seals etc anytime. They don’t even publish the OEM P/Ns – a standard industry practice to maintain revenue and to frustrate OEM cross-referencing and to block this concession. I have a xref here and there is a long one at the end of that page, but it is only partial.
UK 4* is probably close to 100LL.
A search here for “Vliegwerk” turns up interesting hits 
Peter wrote:
So I don’t think the historical fact that the old engines were originally certified with “MOGAS” is very helpful.
They were not — at least not in the case of some Lycoming engines — originally certified with MOGAS. See this article from Lycoming.
Quite. Mogas STCs only came in the mid-80s. And by then, most fuel used by cars was unleaded already.
In any case mogas is a real option in my area and provides a lead free alternative that’s available now. I haven’t seen 91UL anywhere so far.
