Engine Oils and Gear Oils are typically represented by an SAE rating (Society of Automotive Engineers) that appears in the form of a number, e.g. SAE 30. The numbers are based on a standardised system of measuring resistance to flow, or in layman’s terms, the thickness of a liquid. This is typically referred to as Viscosity.
Back in the days when everything was black and white, oils had a single number rating like the above SAE 30. This is referred to as a Mono grade oil for obvious reasons.
The oils viscosity was tested at 100deg C as a generally accepted operating temperature of engine oil. The thicker the oil, the higher the number. For example an SAE 20 has a lower viscosity than an SAE 30, indicating its thinner and flows faster. To illustrate, water would have a very low viscosity, while something like honey would have a very high viscosity.
Why does my car need Engine Oil?
As you may know, engines require proper lubrication in order to have a long and trouble-free life. Engine oil is distributed to the critical parts of the engine via a network of small oil passages, acting as a cushion between the moving metal parts.
If the oil is too thin for a given application, it will not be able to provide the necessary lubrication causing premature wear on your engine as metal-on-metal contact occurs. This can also result in high oil consumption, as the oil will find its way beyond the piston rings and valve stem seals allowing the engine to burn it in the combustion chamber.
Similarly if the oil is too thick, it may not be able to reach the necessary components (some tolerances inside an engine can be as small as two thousandths of an inch!). Thick oil also has an effect on cold start-up lubrication, when an engine is at its most vulnerable. As the oil takes time to find its way through the network of small passages, a thicker oil can prolong lubrication, causing irreparable damage.
Did you know…
A high quality oil filter containing an anti drain-back valve can aid cold start-up by retaining oil in the filter for the next time the vehicle is started.
Should I use thin oil in Winter and thicker oil in Summer?
In the past it was common practice for people to use thinner oil in winter, and thicker oil in summer. However, this is not necessary today.
New technology has developed different alloys in the engine components, and tolerances became smaller, bringing to light the fact that its not only important to know how an oil flows when its 100deg, but also how it performs at colder temperatures, particularly in cold climate countries. As a result we have the “Multigrade” oils we know today.
What are Multigrade Oils?
Multigrade oils are represented by two numbers (e.g. “10w-40”), the first is an SAE rating for cold temperatures, and the second is an SAE rating for hot temperatures. The W stands for “winter” and denotes the cold rating, (10w) with the oil tested at -30deg. This is followed by the hot rating (40) tested at 100deg.
Its important to view both numbers as independent to each other, as they are measured on different scales. Its easy to think that a 10w-40 oil gets thicker as it gets hot, given the low winter rating and the high hot rating, but they are not relative. A 5w-50 engine oil , will be thinner at cold temperatures than a 10w-40 but will not thin out as much as a 10w-40 at 100deg and therefore has a higher hot rating.
Do I need to use the recommended Oil for my Engine?
Manufacturers spend billions of dollars on research and development selecting the correct oils and designing their engines to use specific oil. Some engines even rely on oil pressure to operate various solenoids and switches. E.g., the Toyota’s VVTi and VVTLi systems. For this reason it is absolutely paramount to use the recommended oil for your vehicle. This information can be found in your owners manual and in some cases on the Oil filler cap.
Most manufacturers will also require your oil meet a certain “quality standard” which can be represented in many different ways. The most common is an API and ACEA ratings, both typically referred to by a group of letters. For example “API SL” and “ACEA A5/B4” refer to an oils chemical makeup and additives, as well as its service life. Its imperative you follow the manufacturers guidelines when selecting the correct oil.