Considering the output 4kwh(hopefully continous) it actually is good for fuel consumption! Many modern generators barely manage 1.2 liters per hour for about same kwh or less...
And considering how old it is ,4 liters for 3 hours of 4kwh power/load,it's good enough!
If costs are the problem ,install a propane/LPG kit. Halfs the fuel costs instantly! And the oil+inside of engine run cleaner! Tried and tested on a 7.5kwh generator! Ran all day! Oil stays even cleaner if its synthetic! Summer or winter time/temperatures!