Right engine oil and balanced speed are important
There is a certain grade of oil for every engine. Adding oil that is excessively thick or of wrong quality increases the load on the engine and reduces mileage. Always use the engine oil specified in the vehicle manual.
Speed is equally important. Driving in low gear at very slow speeds increases the RPM, while at very high speeds the air pressure increases. In both cases the mileage is reduced. It is better to maintain a steady and balanced speed.