What we are realy interested in is the fuel consumed to create the horsepower ie the specific fuel consumption.
For a Lycoming IO-520F operating leaned to peak at 74% power the SFC is 0.413 lbs/HP/hr
For the R-3350, which is claimed to be one of the most fuel efficient spark ignition engines, the figure is somewhere between 0.409 and 0.398 depending on whose figures you use
Using the best figures I was able to find for a BMW 328i the figure is 0.446 for highway motoring, where it is only using 24 of its available 250 horsepower
So I would say the Lycomings/Continentals are not doing too bad. Spark advance (electronic?) and GAMI injectors to properly balance fuel distribution between cylinders would be about the only advancements possible IMHO. The stuff hanging off a car engine these days is mostly to satisfy pollutant control. I'm told they can't be operated at LOP because of the catalytic converter and O2 sensor, not much use to aviation where LOP is standard fare for the knowledgeable and properly equipt operator.
as 20l/h of leaded fuel per 100 HP is a bit old fashionned in the 21st century
If you can build an engine that puts out 100HP with a 20 litre fuel burn you've just found a way to print money, as that represents a SFC of 0.313, an absolutely unheard of number. .38 is about the lowest achieved, with .35 being talked about.