Something as simple as motor oil can be confusing because there are so many different oil types to choose from. Not only are there different brands, but there are different types for specific applications. It can be a scary thought to damage an engine by using the wrong oil, especially when many types of oil use similar numeric characters to identify them.
For a motor oil identified as "10W-40," the first number that appears stands for 10 degrees Celsius; the "W" stands for winter. This means that the oil should function and start the vehicle properly when the outside temperature is 10 degrees Celsius (50 degrees Fahrenheit) during winter. The second number determines how thick the oil is while under the heat of a running engine. The higher this number, the thicker the oil will be under heat.
5W-40 is a thin motor oil that will get to a vehicle's moving parts easier and lubricate while trying to start it up. The "40" indicates this is a bit thicker than the average motor oil, which is usually 30 for most cars, and it lubricates the engine more than average while it is hot.
Mechanics commonly use 5W-40 motor oil to support higher mileage engines because it is thicker under heat than the average oil and will lubricate moving parts better inside the motor that have been worn down due to strain or age. Ideally, this oil is used on high-mileage vehicles in climates that experience winters of 5 degrees Celsius (41 degrees Fahrenheit). This oil is reliable for cold engine start-up but may be less reliable for climates that experience freezing temperatures.
10W-40 is a bit thicker than 5W-40 but is still rather thin and works well to lubricate the moving parts of an engine during start-up. This oil's number "40" means it will be a little thicker than average while exposed to the heat of a running engine.
Drivers commonly use 10W-40 motor oil in vehicles with higher mileage because the oil is thicker while the engine is hot; this helps lubricate older moving parts. This oil is meant to start up vehicles in winter climates that experience temperatures of 10 degrees Celsius. The 10W-40 oil is not as reliable for engine start-up in colder winter climates as 5W-40 is.
