Motor oil is the absolute lifeblood of your car, yet so many people misunderstand how the golden liquid of motoring life works. In this blog post, I'll walk you through some pointers.
What does engine oil actually do?
Engine oil has two main jobs: lubricate and cool down all of the moving parts in your vehicle’s engine. The engine turns a reciprocating movement - the pistons going up and down - into the rotational movement of the crankshaft turning the gearbox, and then your wheels.
All those movements in your engine will create friction - the piston has to go up and down along the cylinders of your engine block, and the crankshaft has to rotate inside the crankcase. Because of the constant force those parts are subject to, the contact surfaces between the moving part and the static part have to be kept simple - metal-to-metal. No fancy ball bearings here! This type of metal-to-metal contact forms what's called a plain bearing. The rod bearings between the connecting rod of your pistons and the crankshaft serve as a perfect example.
Now here’s where the oil comes in. When your engine is running, the motor oil is pumped by an oil pump to supply high pressure and flow. The bearing surfaces - such as the one where your piston's connecting rod meets the crankshaft - have holes for the oil to flow in between the surfaces at high pressures so that true metal-to-metal contact is minimal - think about when you try to stop a water hose's water with your finger. A similar mechanism helps your engine stay lubricated at all times!
You've probably heard of water-cooled and air-cooled engines. Chances are, all those engines are also oil-cooled.
Because the oil, for its lubrication goal, is designed to flow everywhere in your engine where there could be friction, it also happens to meet a lot of heat. As the oil flows throughout the entire engine, it helps carry heat away from the hottest parts both to be absorbed by the cooler parts, and to be cooled with external oil coolers.
What do those numbers (like 5W-30) mean?
You've seen them everywhere. 5W-30. 0W-20. 10W-40. What do they mean? Those are referred to as "viscosity grades" or "weights." Coined by the SAE (Society of Automotive Engineers), they decode how thick or “viscous” an oil is. The higher the viscosity grade numbers, the thicker the oil.
Let's decode the grades. The number before the "W" is the cold viscosity - in cold weather, the oil acts as if it is a 5-weight oil. The number after the "W" is the hot rating - in hot weather, the oil acts as if it is a 30-weight oil. This means that, at a colder temperature, a 5W-30 oil is going to act like a 5-weight oil at that temperature, rather than a 30-weight oil at that temperature. This is done through engineering with micro polymers! The graph below shows recommended grades given the weather.
To protect your engine! Colder oil flows slower. When you start a car in cold weather, it will take a while for a more viscous oil to flow through your engine and provide adequate lubrication. Therefore, if your oil can act lighter than its true weight in cold weather, your engine will receive lubrication sooner, which is better for longevity.
Important to note: lots of articles say that a 5W-30 oil is thinner when cold. This is absolutely false! A 5W-30 oil is still thicker when cold, but it is a bit less thick than if it had been a grade 30 single-weight oil. When the oil warms up from the cold to operational temperatures, it will still become thinner, but it won't become as thin as a grade 5 single-weight oil at the operational temperatures.
In short, while the physics of temperature and oil sadly still apply, a dual-weight oil simply mitigates the low-temperature issues of heavy oil while maintaining the high-temperature lubrication performances it can provide.
There are many reasons, but the main one revolves around your oil! Oil lubricates best at an operational temperature, which is quite high: around 90-110C, or 194-230F. You can see this in the graph below.
Therefore, it is important to wait until the oil performs the best to open up that throttle. To let the oil perform the best, you need to heat it up from ambient temperature to the operational temperature. While you should follow your manufacturer's recommendations, a good rule of thumb is that you can run the engine at lower loads (below 3500-4000 RPM for most gas engines, yours may vary) as you begin your drive. As it reaches higher temps, open her up!
Because good things don't last forever, of course. As your motor oil is being heated, cooled, compressed, and mixed with all the wear and tear of your motor, its lubrication properties degrade over mileage and time. The main source of degradation is oxidation. As the oil comes in contact with oxygen, especially in high-heat environments like the internals of your motor, oxygen molecules interact with and change the nature of your oil's molecules. Making matters worse, as the oil comes in contact with fast-moving parts, minuscule metal and plastic particles are mixed in. Imagine those bouncing around in the heart of your car!
Doing your maintenance on time? Use Papertrail to show more for your hard work! Congratulations on making it this far! You now know all the necessary knowledge of your motor oil - and a lot beyond.
If you are putting in this much hard work, you know that documentation is extremely important. If your car is under warranty, a lack of proper documentation on maintenance done outside of the dealership can potentially void any warranty claims. Even if your car is not under warranty, having an organized note is still important for your own accounting, resale value, and future reference.
With Papertrail, you can log each service, note down your intervals, dates, oil used, etc., and even upload pictures and receipts as proof. Give the app a whirl, and keep those oils fresh!
Sources:
https://motorstate.com/oilviscosity-htm/
https://wiki.anton-paar.com/us-en/engine-oil/
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