See Figures 1 and 2
When using engine oil, there are two types of ratings with which you should be familiar: viscosity and service (quality). There are several service ratings, resulting from tests established by the American Petroleum Institute. Use only SE rated oil or SF (for better fuel economy). SF oil passes SE requirements and also reduces fuel consumption. No other service ratings are acceptable.
Oil can be purchased with two types of viscosity ratings, single and multi-viscosity. Oil viscosity ratings are important because oil tends to thin out at high temperatures while getting too thick and stiff at low temperatures. Single viscosity oil, designated by only one number (SAE 30), varies in viscosity or thickness, a great deal over a wide range of temperatures. The single rating number comes from the fact that the oil is basically a single, straight grade of petroleum. A multi-viscosity oil rating is given as two numbers (for example: SAE 10W-40, the W standing for winter ). Multi-viscosity oils slow changes in viscosity with temperature. These changes occur with changes in engine temperature conditions, such as cold starts versus eventual engine warm-up and operation. The double designation refers to the fact that the oil behaves like straight 10W oil at 0°F and like straight 40W oil at 200°F. The desirable advantage of multi-viscosity oil is that it can maintain adequate thickness at high engine operating temperatures (when oil tends to get too thin) while it resists the tendency to thicken at very low temperatures. A straight 30 oil gets so thick near 0°F. that the engine will usually not crank fast enough to start. Because of its versatility, a multi-viscosity oil would be the more desirable choice.
When adding oil, try to use the same brand that's in the crankcase since not all oils are completely compatible with each other.