We all know that in an aircraft with avgas-fuelled engine(s), as we increase altitude, we lean the fuel mixture. To my simple understanding (I'm no engineer), this is done in order that the amount of fuel in the "mix" is decreased in line with the amount of oxygen being ingested from the increasingly less dense air, in order to maintain optimum engine power output and operating parameters.
Not quite correct, you cannot just keep increasing altitude to get optimum engine power once you reach full throttle altitude thats it, unless you bolt on a blower!.
Range is best at L/D Max A of A, which coincides with lowest altitude for full throttle operation.(which explains your question Bose)
Max endurance is best at sea level.
The reason you get away with it going up high (and often actually travel faster) is that with the air being less dense up there, you have less of it to push out of your way as you travel through it. If you have less to push out of your way, then you don't need as much power
If that was correct Cessna 150s would be be fuel efficient in the Tropopause!!
Only (pure) jet engines continue to be more efficient with altitude to the tropopause