This disclosure provides a system and method that eliminates the need for manually calibrating or adjusting a dual fuel internal combustion engine to compensate for variations in composition of a gaseous fuel or other variations, such as ambient or site conditions. The system and method functions by determining an engine load, determining an advantageous gaseous fuel substitution rate from the engine load and speed in addition to an actual gaseous fuel substitution rate, modifying the advantageous gaseous fuel substitution rate by a minimum liquid fuel flow rate, engine protection parameters, and oxidation catalyst protection parameters, and then determining an error term in response to the modified advantageous gaseous fuel substitution rate and the actual gaseous fuel substitution rate. The error term is used to adjust a gaseous fuel control valve.