Offline I exchanged a few messages with another VOC member on the subject of this thread. This reminded me of some fuel experiments I did in the early 2000s that, although they don't touch on advance/retard, still aren't completely off topic.
In the early 2000s I bought an Ertco hydrometer (accuracy 0.15%) and Cannon-Fenske viscosimeter (accuracy 0.29%), both with calibrations traceable to NIST, for measuring the density and viscosity of gasoline. The flow through a jet is a function of these, and both depend on temperature. I did this so I could help a friend who was a highly ranked classic bike racer at the time. The idea behind this was that if he had, say, determined the perfect jetting for his bike on a track in Death Valley in the middle of summer, and his next race was at 10,000 ft. in winter, it would be helpful if he could calculate the main jet he needed so on his first practice run on the new track he would be very close to already having perfect jetting and so could concentrate on learning the track rather than fiddling with jetting.. I still have the "track kit" I assembled of relative air density meter, humidity meter, thermometer, etc., along with a calculator I programmed with the function I determined for the flow rate of gasoline over a wide range of temperatures.
In the course of doing those experiments I measured gasoline from several major suppliers as well as expensive "racing fuel" from a pro shop (whose formulation is always the same). What's relevant for this current thread is the gasoline from the major companies had significant variations in density and viscosity, as well as in the temperature dependence of both, and hence in flow rate. Also, irrespective of the differences between brands the flow rate of a given gasoline varies significantly with temperature, which means the air/fuel ratio also varies with temperature.
Luckily for us, although there is an optimum air/fuel ratio for maximum h.p. under a given set of conditions, internal combustion engines aren't incredibly sensitive to deviations from this ratio. As a result, our bikes still manage to run acceptably well whether we fill up with, say, Shell one time and BP the next, or if we ride our bikes in summer and winter (when the fuel blends are different), or if we are using the same tank of fuel to ride from the hot desert floor to the cool mountain top. Of course, if we gain enough altitude we'll notice a falloff in performance due to the decrease in air density.
Back to timing. If anyone has a link to a credible source of actual data on the effect, if any, of timing due to modern fuels, please post it. I've been looking for such data for several years and haven't found it, although there is plenty of authoritative-sounding but unverified speculation on the web. What makes hoping to find such data problematic is that the U.S., at least, is divided into a fairly large number of different regions with fuel blended specifically for them, as well as differences between summer and winter blends. So, even if someone correctly controlled for all other relevant variables, whatever result they found for the correct timing for, say, Shell Premium gasoline in New York in summer would not necessarily apply to the Shell Premium gasoline in Los Angeles.
Again, my bikes seem to be happy with the factory timing values so I'll continue to use them, but will keep looking for any relevant data that might affect this decision.