Have you ever wondered what all of those numbers really mean on the side of a quart of oil? Those numbers are indicators you should pay attention to because the type of oil you should choose is dependent on several factors including the age of your vehicle, number of miles on your car  or where you live in the United States.

First, what does a number like 10W30 mean? This combination of numbers refers to the weight of the oil. It is a measurement of its viscosity, or how thick it is and how quickly it moves through the engine. Viscosity is resistance to flow.

The first number is a representation of how it flows at cold temperatures. The lower this number is, the easier it will flow when the engine is first started because it is thinner. When a car is first started, thinner oil is better to get it pumping to all parts of the engine.  However, as the engine heats up to normal operating temperature, thin oil can become a problem.

The second number indicates how it flows when it is at normal operating temperature, or when the engine has been running for 5 minutes or more typically at over 100 degrees Celcius. When oil gets hot, it tends to thin out even more so it needs to have some resistance in order to prevent metal on metal contact. For this one, the larger the number, the better it will protect the engine at high temperatures.   

As oil is cycled through your car’s engine, it will continue to heat up even in the coldest parts of the country, therefore its viscosity, or resistance to flow, will also change. So based on all of this, you can see the dilemma. Your car engine needs a thin oil when it first starts (and is cold) but a thicker oil as it runs and gets hot. So what is the answer?

Well, that is where multi-weight oils come into play and that is what those numbers are referring to on the side of the oil you purchase.  Oil weight indicates how it changes at the molecular level to expand and contract with temperature changes that your car goes through during normal use.  So for oil rated as 10W30, it can be thin as you start your automobile, but then modify itself as the engine temperature rises to give it more viscosity.

10W30 vs. 10W40

Based on the first indicator of cold temperature viscosity, both of these weights will perform the same when the engine is first started.  Both will pump oil easily through the engine quickly.

However, the second number indicates where they diverge in performance.  A 30 weight oil is very common for most newer cars and considered ideal under most normal operating circumstances. If you had to choose the best all-weather performer, this weight would win for most situations.

However, if you live in a warmer region and drive an older car, switching to 10W40 often solves low oil pressure issues and is the best option because it provides more protection for older engines. Over time engine parts get worn and the thicker oil will fill in some of the gaps preventing leaks throughout the engine, thus reducing oil consumption.  

As a car ages, you are never going to stop all leaks because gaskets and seals wear out over time, but a higher viscosity oil at higher temperatures can help.

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How Often Should I Change My Oil in My Car?

What Type of Oil Should I Use with my Car: Understanding Oil Viscosity

The old recommendation for changing it every 3000 miles was largely driven by quick change auto shops looking to keep you coming back. If you have an older car, it will leak more oil, thus requiring you to add a quart more often, however, there is no real reason other than paranoia to change it every 3000 miles unless you are driving in really harsh conditions such as off-road areas.  Even the car manufacturers do not recommend an interval at that frequency.

One thing you should consider if you hate getting your oil changed is to go with a synthetic oil like Mobile-One. Combined with a good oil filter, you can easily go 6000 – 7500 miles on normal use between oil changes. Some even suggest you can go as many as 10,000 miles between oil changes, but I would not push that especially if your car is more than five years old for all of the reasons listed above.

Synthetic oils come in the same weights as normal oils and all of the major oil brands have their own version.

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Final Thoughts

What Type of Oil Should I Use with my Car: Understanding Oil Viscosity

Most new cars are designed to run 10W30 year-round in most climates so that is what you should be using if you own a car less than 10 years old. If you live in an extremely cold climate, as in Canadian cold, you might even try a 5W30 weight at least during the winter months, since it will take the engine longer to heat up.

However that is generally reserved for high revving newer engines since it is so thin and heats up and breaks down quickly.  So as a general rule, unless you live in the Arctic, you will probably be ok with 10W30 for cold weather.

If you live in warmer regions in the south or southwest and drive a car that is at least 10 years old or over 100,000 miles on it, as a general rule of thumb you should be using 10W40 year round.

Yes your engine is still “cold” when it first starts but that is accounted for with the 10 rating. As it quickly heats up, you will be dealing with higher engine temperatures in hotter regions so the higher weight oil will provide more viscosity, and thus protection for the engine. In short, you will be putting less oil in it because you will have fewer oil leaks throughout the engine.

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