Oil Change - Knowing Your Viscosity

When you take your car in for an oil change or when you find yourself in the aisle of an auto supply store, you'll be faced with a lot of numbers on the labels that you might not understand. If you are like most people, you have the false assumption that the numbers associated with motor oil choices have to do with the type of car you drive. Those numbers, however, are indicative of what type of climate conditions in which your vehicle is operated.
Not only does viscosity have nothing to do with your car itself, it also has nothing to do the quality. The number has everything to do with the weather and/or climate conditions in which you most operate your vehicle. It is important to know what type of weight you are using for several reasons.
If you purchase a car off of a local car lot, it should probably be relative to the local weather conditions, and you can usually find an indication of what type is in the car by looking inside the frame of the driver’s side door. Do not just assume, however, that the right viscosity has been handed over to you. Even new cars get shuffled around from lot to lot, and it is possible that a car has come from, say Oklahoma down to a lot in Mississippi where the viscosities preferably would be different. 
It is also important to know your own car’s viscosity while you are in travel situations. If for some reason you find yourself traveling in another climate condition when you decide to get an oil change for your vehicle, it would be a reasonable assumption for the technician to put the locally recommended viscosity into your car. This of course is not going to hurt your vehicle, but for optimal performance of your engine, you want to be using the correct engine lubricant in accordance with your surrounding climate conditions.
Now which viscosity do you need? A single number assignment such as the types SAE10 or SAE30, which are also called straight weights, are meant for those who drive where temperatures are pretty consistent. Lower numbers are affiliated with colder weather, thus the higher the number is the warmer weather should be. 
Multi-viscosity assignments such as SAE 5W-40 are a little more complicated. The W in this scenario means “winter grade,” and you should look for these weights if you drive in a climate that experiences winter. The lack of a W in the number string means that the oil is meant for warmer climates. The previsously mentioned example, SAE 5W-40 , would be suggested for a driver who operates in frigid winters and extreme summers, perhaps a mid-westerner. If, during an oil change, you choose the wrong viscosity for your local climate, your car's integrity may be in trouble.