[Unique427]

Here's an abbreviated version of the link posted by OLDDOG.

AvGas 100LL or "100 Octane Low Lead "
TEL (Tetra-ethyl-lead) is added to raise the octane. When used in a motor vehicle this fuel will leave a lot of lead deposits in the motor. 100LL has a high lead content (0.5 grams per litre), even higher than leaded race fuels.
The deposits left when TEL(lead) is burned are corrosive and damaging to valves, valve guides, valve seats and cylinder heads. Lead deposits will also block oxygen sensors and foul spark plugs even after only a short use.
100LL has a chemical package for performance at high altitude, not the best thing for motor vehicle performance at ground level.
AvGas is blended for large-bore, long-stroke, low RPM engines at high altitude.
AvGas' higher octane is useful but smaller-bore, shorter-stroke, high RPM engines will perform better on racing fuel or quality octane boosters.
AvGas has lower volatility so when used in proportions higher than about 40%, part-throttle drivability and cold starts may be compromised.
AvGas has a lower specific gravity so it will require a change in air-fuel ratio calibration for best engine performance.
100LL is blended with a high percentage of aromatics causing reduced throttle response which is not an issue with an aircraft engine but certainly an issue in a high-performance automotive engine.
These high levels of aromatics will also damage rubber components in automotive fuel systems such as fuel lines, fuel pump seals and injector washers.