Thanks Peter, The final piece of the puzzle just dropped in!
The precipitate is most likely Lead bromide - the stuff that makes your exhaust white. I evaporated some avgas in the dark and got no precipitate so the precipitation is caused by sunlight. What Avgas has got that 95 octane hasn't, and is photosensitive, is tetra ethyl lead and ethylene dibromide.
A little research yielded this: (URLs on machine at work so don't have them now)
Ethylene dibromide, a clear liquid, decomposes under the influence of light to form ethylene glycol and free bromide ions which are pretty reactive.
Tetra ethyl lead, another clear liquid, decomposes under light to give ethylene(I think) and free lead.
Lead atoms plus bromide ions react readily to produce lead bromide which is a white solid at normal temperature. This is the reaction that also happens in your combustion chamber where it does its magic. It then goes down your exhaust to make it white.
It seems sunlight is mimicing the conditions in the combustion chamber which activate the lead chemistry to give the precipitate in the fuel.
The identifier dye turns the white powder pale blue.
It doesn't happen with 95 octane as they use heavier benzene or its derivatives to increase the knock rating - ? the oily residue you got, Peter.
Any real chemists out there confirm this?
Last edited by Gaseous; 25th Apr 2005 at 20:19.