Thanks for the info so far.
I am an engineer by profession and have a very good understanding of the variability of Mogas. I have no doubt whatsoever that Avgas is a far preferable fuel and would use it if I could sensibly obtain supplies. Price is not the issue. When you tank capacity is just 34 litres useable and it consumes 12 litres to go and get some by air or requires 60 minutes round trip by road, things are not so simple......
What I'm trying to find out is what is the residue? Gum from evaporated fuel? Corosion product? Decay products from a slowly disolving fuel tank?
The tank is 1960s vintage so testing resin samples is not a realistic option. I am considering lining it with a modern sloshing compound, though there does not seem to be much good information on the suitability of these compounds for use with modern mogas.