Captain WW, I understand your reply but it does not quite address the question I asked.
I understand that a designated fuel supplier would not be associated with supplying non-compliant fuel, which is how they would view mogas.
My question was the extent to which the term 'sole fuel supplier' is applied. Is it appropriate to consider an owner/operator sourcing fuel (mogas) only for their own use as a 'supplier', surely that is unreasonable in the extreme?
Recent replies indicate that it would be wrong to consider mogas/avgas to be completely interchangeable, if the designated supplier is not able to supply the required grade (because they choose not to handle mogas) then they are not loosing a sale by restricting customers who would choose to source their own mogas.