How to choose the right oil for your engine

How to choose the right oil—Learn what oil ratings and viscosity mean when choosing an oil

Choosing the correct engine oil is crucial for problem-free driving and maintaining the longevity of your engine. With various options available, selecting the right oil might seem daunting. However, understanding oil ratings and viscosity can help simplify the process significantly. Let’s start with viscosity because it’s the single most important factor to help you choose the right oil.

The right oil viscosity is determined by the carmaker, not your neighbor

Carmakers choose the recommended oil viscosity based on the design of the engine and the engine’s bearing clearances. Modern engines are designed with smaller clearances and require thinner viscosity oil so they can achieve maximum fuel economy. If your owner’s manual recommends 0W-20 oil and you install 10W-40 oil, the thicker oil won’t flow as quickly, won’t remove the heat as well from bearing areas, and will create its own heat due to its thicker body. The end result will be more engine wear.

It order to lubricate and cool, the oil must flow quickly

Oil pressure prevents metal parts from touching

when the engine starts and not thin out too much when the engine reaches operating temperature. That’s why choosing the recommended viscosity is so important. Many self-proclaimed oil experts recommend using a higher viscosity oil than the carmaker’s recommendations.

Look at the diagram to the right. A typical crankshaft bearing clearance is only .0010″. So the clearance is just .0005″ on each side of the crankshaft. If you use an oil that’s thicker than the recommended oil, you’ll actually create more crankshaft and bearing wear.

Oil viscosity also affects the variable valve timing mechanisms in your engine

Most modern engines have variable valve timing (VVT) mechanisms to obtain maximum performance and fuel economy. The VVT mechanisms advance or retard camshaft timing by pumping oil into advance or retard chambers in the VVT phasers. The ECM changes valve timing by pulsing the oil into or out of the chambers. If you use the wrong viscosity oil, too thick or too thin, that will affect how much oil enters or leaves the chambers, therby resulting in abnormal valve timing and a check engine light.

Next, let’s consider the effect of altered motor oil viscosity on cold-starting performance

Let’s use a 2005 Ford Taurus. The Ford spec calls for a 5W-20 synthetic blend motor oil. I’ve pulled up the specs for Valvoline’s Durablend synthetic blend motor oil because it meets Ford spec WSS-M2C153-H, but you can use any brand that meets that spec.

At 40°C the oil viscosity for 5W-20 is 45.78(cSt). However, a 10W-30 Durablend at the same 40°C temperature has a viscosity of 71.45(cSt).

Cold starts reduce oil life

That’s an increase of 56%! Since the majority of engine wear occurs at cold startup, changing from 5W-20 to 10W-30 causes a significant decrease in flow rate. So you experience more cold engine wear. That factor alone should convince you not to vary from the manufacturer’s recommendations. But it gets worse.

Now let’s talk about Viscosity Index, or the change in the oil from one temperature to the next. Here’s an official definition of VI from CASTROL:

“The VI of an oil is calculated from its kinematic viscosities at 40°C and 100°C, and the resultant value indicates the temperature/viscosity relationship of that oil. An oil with a high VI experiences smaller decreases in viscosity as the temperature at which it is working increases than does an oil with a lower VI. Conversely, as the ambient temperature decreases, the viscosity of a high VI oil will increase at a lower rate than that of an oil with a lower VI.”

Now, let’s talk about motor oil categories

Motor oil ratings are determined in coordination between the oil industry and the carmakers. The International Lubricants Standardization and Approval Committee (ILSAC)ILSAC was founded to define the need for lubricant specifications in collaboration with the Tripartite system, comprised of the American Petroleum Institute (API), the Society of Automotive Engineers (SAE), and the American Society for Testing and Materials (ASTM). Together, they established the Engine Oil Licensing and Certification System (EOLCS). In the U.S., the categories are written by the American Petroleum Institute’s (API) Lubricants Standards Group.

The current ILSAC Specifications for motor oil are:

GF-6A Current Introduced in May 2020, designed to provide protection against low-speed pre-ignition (LSPI), timing chain wear protection, improved high-temperature deposit protection for pistons and turbochargers, more stringent sludge and varnish control, improved fuel economy, enhanced emission control system protection and protection of engines operating on ethanol-containing fuels up to E85.

GF-6B Currently Applies only to oils having an SAE viscosity grade of 0W-16. Introduced in May 2020, designed to provide protection against low-speed pre-ignition (LSPI), timing chain wear protection, high temperature deposit protection for pistons and turbochargers, stringent sludge and varnish control, improved fuel economy, emission control system protection and protection of engines operating on ethanol-containing fuels up to E85.

SP is the current API service classification. Introduced in May 2020, designed to provide protection against low-speed pre-ignition (LSPI), timing chain wear protection, improved high temperature deposit protection for pistons and turbochargers, and more stringent sludge and varnish control. API SP with Resource Conserving matches ILSAC GF-6A by combining API SP performance with improved fuel economy, emission control system protection and protection of engines operating on ethanol-containing fuels up to E85.

© 2012 Rick Muscoplat