In Switzerland we get Shell's 100 Octane and BP's 98 'High End'-whatever fuels. I get about 50 miles more out of my '89 760 GLE V6 in Summer but in Winter with all the cold starts and stuff MPG is as low as always. I don't know the MPG formula but maybe this helps:
Shell 100 in Summer: 600km equal 70litres, versus regular Shell/BP/whatever fuels@95 octane when 500-550km equal 70 litres.