Engine oils come in different grades as mentioned above. The grades are determined on the basis of their viscosities. The viscosity of an oil is measured by its resistance to flow. Different engine oils have different viscosities at room temperature and they also react differently to temperature changes. So, selecting the right grade largely depends on your geographical location and the climate of the area you are driving in. The grading is always mentioned on the body of the container, for example, a 5W-30 or a 10W-30. The ‘W’ stands for Winter; this measurement is related to how oil flows when it is cold (the temperature at engine start-up). Therefore, the smaller the number, the better it will flow. This is important to understand that these lubricants naturally thicken as they cool, and thin as they are heated.