Ghulam Haider

I BOUGHT 2001 LEXUS RX 300 A MONTH AGO. THIS IS THE FIRST CAR IN MY LIFE. SEVERAL THINGS I NEED TO KNOW. WHAT IS MEAN BY ENGIN CHECK SIGN, WHAT IS O/D ON THE GEAR, WHEN SHOULD I CHANGE OIL. MY CAR CONSUMES 10 MPG GAS IS IT POSSIBLE TO DECREASE THE GAS CONSUMPTION BY CHANGING SOMETHING IN ENGIN. SHOULD I WASH THE ENGIN OR NOT?