If we look back many years to the piston engine airliner era, we find that, if the weights are identical, and further considering that we are talking about altitudes higher than 10,000 msl, the fuel consumption, for the exact same power (BMEP) setting (and these engines were nearly always cruised at the same BHP, for enhanced engine longivity), will be the same for 10,000 msl as it would be for FL210.
However, the true airspeed will increase at the higher flight level, on the order of two knots per 1000 feet (approximately).
Piston engines (in general) are far less affected with regard to fuel consumption that are turbine engines.