Agreed. Switch to good quality plugs and synthetic engine oil. this will improve the milage.
at 40K checkup, i think you should also get your tyres inspected, rotated and balanced. also get the car aligned again.
check for brake disc & shoes wear.
regarding gear oil change, the interval should be mentioned on your service manual along with suggested specs of the oil.
Also probably you should change the coolent as well (if appropriate).