For Formula 1 cars it is 5mg/l. Old four star leaded has about 840mg/l but is now banned. In the UK low lead petrol is available from a small number of specialist garages for classic cars which cannot run on unleaded and contains about 75mg/l.
So while none is the best, this is a very small fraction in comparison as to what it used to be, and it also is to cover cross contamination purposes.