With so many options on store shelves when it comes to oil for your car's engine, it's easy to become confused. Engine oil is classified by weight and assigned a number. Once you understand this number, it's easier to understand which oil does what.
5W40 motor oil is an oil ideal for use in vehicles that operate in cold climates. The number "5" in 5W40 represents the oil's viscosity in cold temperatures. The lower this number is, the thinner the oil is, and thus more suitable for cold-weather use. The number "40" represents the oil's viscosity in hot temperatures. The higher this number is, the thicker the oil will stay when it is hot.
10W40 is a motor oil that is slightly worse for use in cold temperatures than 5W40 oil. Because the number "10" in 10W40 is higher than the number "5" in 5W40, the former is slightly thicker in cold temperatures. Both oils are equally suitable for use in hot climates. When oil is thick, its viscosity is reduced, putting more strain on your motor.
What to Use
Your vehicle's owner's manual, as well as your oil tank cap, will indicate which oil should be used for your vehicle. It will not do immediate damage to change the recommended oil weight, and this may be necessary if you drive in an extreme climate. If you believe you should change the weight of your oil, consult a licensed mechanic.