Propulsion is a function of moving air.
Any sort of mechanical propulsion regardless of the fuel has an optimum altitude.
Under standard atmospheric conditions in an atmosphere with no wind it would depend on chosen system ( propeller, ducted fan) and wing design.
This would be an ideal cruise altitude but in reality many more factors need to be considered.
Winds aloft, sector length, GPS direct free-flight vs airways, regional and seasonal weather avoidance blah blah blah blah.
If it’s not fuel efficient ( energy efficient) to climb to FL380 now then it won’t be if we choose another fuel source.