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The Science of Motor Oil Viscosity

The Science of Motor Oil Viscosity
November 11, 2021

Choosing the right oil for your motor can be an overwhelming task when you consider just how many options there are. And to actually understand what you are buying when you grab that 15W-40 bottle off the shelf is an entirely different battle.

The Rating System

Motor oils use a rating system developed by the Society of Automotive Engineers (SAE), to classify oil by viscosity. There are a few different ways to categorize motor oils. Straight weight oils have a set viscosity, a single grade. These oils are uncommon in modern vehicles but some small engines and older model vehicles will sometimes recommend a straight weight oil. Straight weight oil will be named something like SAE30, for example.

Oil spread pattern biased on viscosity

This photo demonstrates how oil behaves at verifying viscosities. The lower the weight, the quicker is spreads out on this angled table.

Aside from straight weight oils, there are also multi-grade oils. This is thee most common type of oil used in modern automotive engines. This means that the oil flows differently at various temperatures. This may sound confusing, but it’s actually quite simple to understand when you break it down.

The first number in the series represents the cold weather viscosity. The oil’s viscosity determines how easily oil pours at a specified temperature. Thin oils have a lower viscosity and pour more easily at low temperatures than thicker oils with a higher viscosity. The ‘W’ stands for “Winter” so it’s easy to remember the first part of a multi-grade oil’s name is in reference to cold weather viscosity. The second number represents the viscosity at operating temperature. As an example, a 15W-40 oil has a viscosity grade of 15 at 0°F and a viscosity grade of 40 at operating temperature (212 °F)Remember, the higher the number, the thicker the oil. So, a 15W-40 oil will have a slower flow rate than a 10W-30.

Testing Equipment

The dominant procedure for performing viscosity measurements is an ASTM D445 (centistoke) test. The sample is placed in a heated bath at a set temperature (40°C or 100°C), after allowing the test material to equalize to the test temperature the viscosity can be measured. A capillary tube is used to measure viscosity. After the test material comes to test temperature, the oil is drawn above the start mark. The oil is then timed how long it takes to drain to the stop mark. This time is used in a formula to determine the viscosity.

  1. Warming area for test fluid
  2. Testing holding area
  3. Start mark
  4. Stop mark

oil viscosity testing equipment

Follow the Manufacturer’s Recommendation

At the end of the day, it is always best to follow the manufacturer’s recommendation for the weight and whether to use a multi-grade or straight weight oil in your motor. The recommendation is based on both ambient temperatures and operating conditions. Failing to follow these recommendations could cause lasting damage to the motor, decrease in efficiencies, and would likely void any warranties you may have. Using an oil that is too thick will require the motor to “work harder” to move that oil throughout the system causing a decrease in efficiency. Using an oil that is too thin, can lead to a drop in oil pressure and ultimately causing excessive wear and other problems that are easily avoidable.

Contact Hot Shot’s Secret for more information

Hot Shot’s Secret offers multiply grades and viscosity of oils for a wide array of applications. GREEN DIAMOND FLEET DIESEL ENGINE OIL is a synthetic Group III base oils in our full synthetic oil and a Group II/III blend in our semi-synthetic oil, used in most common diesel applications. BLUE DIAMOND PAO ENGINE OIL is a poly-alpha-olefin (PAO) Group IV synthetic oil that requires a very small amount of viscosity improvers to meet the weight specifications. The infusion of our patented FR3 Nano-Technology and CK-4 additive package provides unsurpassed levels of longevity and performance while keeping the oil cleaner longer and providing better mileage and horsepower. Contains high detergency and exceptional oxidation and thermal stability that prevents deposit formation and oil breakdown. GRAY DIAMOND EURO ENGINE OIL is a premium full synthetic, mid SAPS engine oil engineered to provide superior performance and protection for gasoline and diesel European vehicles.

Moving away from traditional oil applications. BLACK DIAMOND OUTLAW PAO DIESEL ENGINE OIL is formulated to provide extreme wear protection for off road and street/strip applications, and is specifically developed for non DPF applications. Non DPF applications allow a much higher level of zinc in the oil, resulting in up to 3x the amount of wear protection. Infused with our patented FR3 Nano Technology, this oil provides the absolute best level of protection to your diesel engine, as well as an incredible 14 TBN additive package. The ADRENALINE R-SERIES RACING OIL were formulated for the toughest applications in racing with a wide range of weights available.

Like all Hot Shot’s Secret products, you can rest assured knowing that your purchase comes with our money back guarantee. If you have questions about our engine oil, feel free to contact our team for more information. When you’re ready to buy, choose a size and add it to your cart, or call 800-341-6516 for information about bulk-quantity discounts.