Well to be 100% honest, running 25:1 of the semi synthetic with 98 octane i have noticed this to a degree.
When I bought my grey 1989 Standard 2 from the tip, the exhaust port had a reasonable amount of carbon in it. I have probably mown the lawn 10-15 times with it (takes 500ml of fuel each regular mow and around 1 litre for a heavy mow) and I ripped off the exhaust the other day (to check there was a clog or if actually was any wire mesh left in the exhaust because it was getting noisy hard to start) and there was less carbon in the exhaust port and more in the muffler - a lot came out when I stuck some wire in there.
Maybe it is because the 98 runs a bit hotter it allows for a bit of erosion of deposits, and the better oil means less is redeposited
Either way, I have honestly noticed a bit of a difference