Hi Deejay59, welcome to Outdoorking.
Like some other engine manufacturers, Briggs and Stratton recommend emptying the tank and running the engine dry before storage, or if the storage is only for the winter season, adding fuel stabiliser to prevent water absorption and gum formation.
The main mechanical benefit of the Tetra-Ethyl Lead (TEL) in old-time petrol was that it decomposed into metallic lead during combustion and the lead tended to coat the hottest parts of the valves, substantially reducing wear to valve guides and valve seats on 4 strokes. The TEL did far more harm than good mechanically however. The lead it formed tended to short-circuit the spark plug, so extremely chemically active chemicals, known as "lead scavengers", had to be added to the TEL to limit that problem. The bromides and halides the scavenger produced after combustion then ate the exhaust system. Have you noticed that exhaust systems last more or less indefinitely these days, compared with five to ten years in the old days? And of course spark plugs no longer require periodic cleaning.
Lead may help to lubricate the piston rings in 2 strokes, especially when crossing the ports, but I haven't heard of it having significant effects. Essentially, TEL was a desperate move invented in the days before catalytic cracking was used in oil refineries, in order to raise octane levels. Without TEL, uncracked and un-hydroformed gasoline was very lucky to reach 60 octane, so compression ratios had to be so low that fuel economy was appalling. With modern refining techniques a reasonable yield of petrol can be obtained at up to about 90 Research octane. Of course given the chance, refiners will still dope the fuel with octane improvers so as to produce more petrol and less kerosene, but I wouldn't hold my breath waiting for them to get permission to put appallingly toxic substances like TEL in the fuel again.
