The best advice is to follow the manufacturer’s recommendations for both type and weight of motor oil. Most manufacturers will list several weight or viscosity options for your engine depending on average temperature of your driving environment. Typically, in our area of the country we fall into 32 degree Fahrenheit and above operating range for all but a few days of the year. Most modern engines will run 5W-30 or 5W-20 motor oil. Lower viscosity oils are used as the factory fill oil on most new cars because it pumps through the engine more quickly after start-up (important for keeping overhead cams properly lubed), flows through the smaller passages and tighter clearances of today’s engines, makes cold weather starting easier and reduces fuel consumption. There are some European manufacturers that use a 5W-40 and most Diesel engine applications use a heavier 15W-40 specialty blend. Using heavier weight oil than is specified can cause reduced performance and lead to premature engine wear and even failure. The quality of the oil is equally as important as the viscosity. Most applications can use conventional motor oil which means it is refined from crude oil. Not all motor oils are created equal though. The quality of the oil depends on how well the base stock is refined (cleaned) and what is the quality and levels of the additive package that the manufacturer mixes with the base stock.