Lead has always been allowed for off highway use, but most fuel is sold for on highway use and so most off highway machines are designed for unleaded as a user convenience further reducing the use of leaded fuel. Racing fuel is a pretty minor case and aviation gasoline is something less than 0.5% of gasoline sold in the USA.
The lead acts to prevent erosion of the exhaust valve seat through micro welding during hot operation, "hardened" valve seats are a totally different alloy that chemically doesn't weld with the valve steel. Simple hardening would probably be lost due to hot operation. the issue is minor in low compression lightly loaded engines where the valve doesn't get too hot and a big issue in higher compression or heavily worked engines.
Most fuel for surface vehicles land or sea is only loosely regulated and the the bulk of a formula is up to the refinery/customer including the choice of octane levels.
Avgas 100/130LL(blue)
http://www.aviation-fuel.com/pdfs/avgas100llspecsastmd910_2011.pdf is a low lead formula with enough lead to prevent valve erosion, not enough lead to act as a stand alone octane booster(they start with a good base fuel). Each grade of Avgas has to meet a strict pre-approoved specification, to the point there really are no "brands" because their is no significant difference from refinery to refinery. Especially as the use of particular fuel is required as part of an an aircraft's type certificate.
For example alcohol is explicitly not allowed in avgas for reasons that don't have a big impact on auto fuel, and there is a minimum shelf life from the time it leaves the refinery if properly stored.(2 years as I recall) The general chemical makeup the base stock is also controlled.
100 is on the "aviation lean" knock performance scale (roughly matches motor octane) it is not street octane, this is closer to 105 for street gas. The 130 is the "aviation rich" knock performance scale which has no equivalent test in street knock tests as the test requires the use of lead and is meant to simulate a full power takeoff and climb and or supercharged engines.
I buy 100LL for my old 1960s tractor, it is only 7.5:1 and rated for about 90octane but the continuous power load is high so the valves still get fairly hot. I just haven't had the head off to put in new valve seats, and with the occational use it gets I actually end up saving money with avgas because along with valve seat cost it sits for long periods, where oxygenated highway gas will go bad and gum up the carb too. (100LL is only a dollar more per gallon than highway92)
I use non-alcohol unleaded for newer yard equipment, because it also stores resonably well and equipment my use is sporadic. (Not as long as avgas, but better than gasohol.) Avgas is not only non-oxygenated it actually contains a dose of anti-oxidants.
Note you need to consider your crankcase oil when using leaded gas, some oils may not be designed to dissolve the lead blowby, just keep a closer eye on it, maybe use aviation oil (shell, phillips66, or mobil) or do more changes.
