"Today's gasoline" (OP's terminology) covers a host of evils.
(A) The stuff added in the late 1970's to increase volatility is one issue.
(B) The lack of lead is another issue.
(C) And, of course, ethanol yet another.
As mentioned earlier by sstock, the lack of lead is not a large problem with collector vehicles (unless they are tractors which regularly participate in tractor pulls, or are used on the farm). Racing, pulling a heavy trailer, or other environments which significantly load the engine can mean issues with lead-free gas and engines not designed for them. But for the general enthusiast, few, if any problems.
The higher volatility, and the lower BTU content of the ethanol cause more problems than the lack of lead.
I personally had one collector car that got started about once a year, where the sediment from the modern fuel evaporating STOPPED UP THE INPUT TUBE IN THE SENDING UNIT, REQUIRING THE TANK TO BE DROPPED AND A NEW SENDING UNIT INSTALLED.
Sediment from evaporation IS an issue, especially with collector vehicles.
Air-cooled vehicles suffer from the use of E-10 with the reduced energy content, as they tend to run hotter. My John Deere lawn and garden tractors will run all day on real gasoline; but put E-10 in the tank, and if the ambient is above 90 degrees F, will start to sputter, and eventually stall after 30 or so minutes. Of course, at my age, maybe not a bad thing to have an excuse to get out of the sun after 30 minutes
And starting the small engines after they sit unstarted for 3 to 4 weeks is an exercise in futility. I finally started buying starting fluid by the case; and simply start the small engine on the ether. Saves the battery and starter on electric starts, and my temper on hand crank engines. I do this on everything from a weed eater on up to a 450 HP collector vehicle, and have now for about 10~12 years with no known issues from using the ether.
The very best "insurance" for dealing with modern fuel is to start and run the engine at least once a week!
Jon.
