Motor oil "thins as it heats and thickens as it cools," according to Popular Mechanics. Engines need oil to be thin when starting, when they are cold, and thicker oil once they heat up. The first number in the oil weight (the 10 in 10W-30) identifies the temperature in which oil is effective. A lower number is better. The second number is the viscosity rating, and a higher number represents a thicker oil.
Difference Between 10W-30 and 10W-40
Both 10W-30 and 10W-40 motor oil can be down to zero degrees Fahrenheit. Popular Mechanics says 10W-40 motor oil stays thicker at higher temperatures. Because oil tends to get thin at higher temperatures, increased thickness can be very valuable. Oil that is 10W-40 will stay thicker at higher temperatures than 10W-30 oil.
Choosing the Right Oil
It is important to use the auto manufacturer's recommended oil weight, according to Popular Mechanics and Auto Education. The recommendation can be found in the owner's manual.
Motor oil comes in a number of weights. The weight that a car uses can make a difference in how well a vehicle drives, as well as fuel efficiency. Two of the most common oil weights are 10W-30 and 10W-40.