As for using 100LL in classic vehicles, one thing not yet mentioned above is that they may become more difficult to start in Winter (due to the lower vapour pressure of avgas compared to winter grade 4*)
Which is why you should only add it to unleaded petrol, about 1 to 4.5, which brings it up to the level of the old 4*. In fact it's probably more like 5* at that ratio, due to the better basic formula of today's fuel.
Once you've got enough TE lead in the fuel to prevent pinking, there is no point adding any more; it will cause more problems than it's worth, such as plug contact whiskering. So AVGAS is best used as a fuel additive, rather than a replacement fuel by itself.
Obviously, for off-road use only, to remain legal.