The Engine Oil Bible
Viscosity
Eeeewwww! Sticky!
Viscous, actually.
When you see 10W50 oil, you know (from reading my pages) that the '10' and the '50' refer to viscosity of the oil at different temperatures. But what do those numbers actually mean? Well, they're called 'Saybolt Seconds' and are measured using a Viscometer. There are three different types of viscometer though, and they all look like something you'd find on the set of a Flash Gordon episode. ("must....kill.....Flash.....Gordon........")
First up we have the Redwood Viscometer, also known as the Standard British viscometer. Redwood Seconds refer to the number of seconds required for 50ml of the oil to flow out of the device at a predefined temperature. The instrument is available in two sizes: Redwood type-I and type-II. When the flow time exceeds 2,000 sec, the type-II must be used. I wouldn't recommend putting 10W2000 in your car though ....
In true VHS vs. Betamax fashion, the industry can never settle on one standard, so there's also the Engler Viscometer. This measures in Engler Degrees, rather than Redwood Seconds, and is preferred by the rest of Europe. On the Engler Viscometer, the reading is the time (in seconds) required for 200ml of the oil to flow through the device at a predefined temperature.
The conversion of Engler degrees to absolute units requires an appropriate table, a degree in rocket science and an intricate knowlege of fluid dynamics.
Alternatively, an oil page written by a garage hack will do the trick. To wit:
For liquids having a viscosity of 100 centistokes or more the Engler degree is roughly equal to 7.6 centistokes.
So for kinematic viscosity, the formula is:
kinematic viscosity in centistokes (cSt) = Ct
where C is the calibration constant of the particular viscometer and t is the observed time of flow. The value computed by this formula is reported in centistokes, units of kinematic viscosity.
I don't expect you to understand that - frankly I don't but it looks good when I put the information on the site, so if you're going to email me about anything, please don't ask about Engler degrees.....
Finally, in the category of "America" = "The World", there's what is affectionately referred to as Saybolt Viscosity seconds - the term I used at the top of the page. For the Saybolt Viscometer, the amount of oil to be measured is 60ml. There's two types of Saybolt Viscometer, as with the Redwood system. Type-I is called the Furol Viscometer, Type-II is called the Universal Viscometer. "Furol" is a made-up word based on "Fuel and Road Oil" - ie. that's what's used to test the oil you put in your car. The Universal Viscometer is used for other industrial lubricants and oils, and has largely been superceded by kinematic viscosity methods - those performed using the type-I system.
You'll notice for all of the above, they measure time for a predetermined temperature. Under these classifications, the winter grades of 5W, 10W and 20W are determined by the oils' viscosity at 0° Fahrenheit (-18°C), while grades 20,30, 40 and 50 are determined by its viscosity at 212° Fahrenheit (100°C). Those are the predetermined temperatures.
On to the results, then.
Saybolt viscosities are reported as the number of elapsed seconds indicated by the timer. For Saybolt Universal viscosities, the units are Saybolt Seconds Universal (SSU), and for Saybolt Furol viscosities, the units are Saybolt Seconds Furol (SSF). For a given oil, the Saybolt Universal value will run about 10 times as high as the Saybolt Furol value at the same temperature.
Redwood and Engler viscosities are also based on the time of flow and are reported as "Redwood seconds" or "Engler degrees," as the case may be. In all instances, the test temperature is reported along with the corresponding viscosity. In case you're at all inclined, here's some sample values for common oil ratings.
| Viscosity equivalents at 212°F - summer ratings | |||
|---|---|---|---|
| Kinematic (Centistokes) |
Saybolt Universal (Seconds) |
Redwood (Seconds) |
Engler (Degrees) |
| 1.8 | 32 | 30.8 | 1.14 |
| 2.7 | 35 | 32.2 | 1.18 |
| 4.2 | 40 | 36.2 | 1.32 |
| 5.8 | 45 | 40.6 | 1.46 |
| 7.4 | 50 | 44.9 | 1.60 |
| 8.9 | 55 | 49.1 | 1.75 |
| 10.3 | 60 | 53.5 | 1.88 |
What about 0W oils then?
Good question. Given that you can't have 50ml, 60ml or 200ml of oil flowing through any size hole in zero seconds, what on earth does the 0W rating denote? Well it's a special case denoting a difference in the 'pour point' of the oil. Most 5W oils have a pour point at -40°F (-40°C) The base oil is the same in 0W40, but it's pour point has been lowered even further - sometimes to as much as -50°F (-46°C)
Pour point is 5°F above the point at which a chilled oil shows no movement at the surface for 5 seconds when inclined. This measurement is especially important for oils used in the winter. A lot of manufacturers tend to quote pumping temperature rather than pour point. Pumping temperature is the temperature at which the oil will pump around the engine and maintain adequate oil pressure. This is typically 20°F above the pour point - ie. 25°F above the point at which the oil is basically a gel.
So 0W oils don't flow through a viscometer in zero seconds - they rate at 5 seconds like a 5W oil, but they will be pourable at a much lower temperature. The bottom line then is that if you think your car is ever likely to see a cold morning in the -45°F (-43°C) range, you should be considering 0W40 oil. If not, 5W40 will do. Note that at -45°F, you'll probably have more to worry about than your engine oil - like your radiator fluid, brittle tyres, frozen locks, permafrost on the windscreen etc.etc.etc.......
Cold Crank Simulators
In addition to measuring the viscosity of oil, motor oil manufacturers typically also use a cold crank simulator (CCS) to measure the apparent viscosity of the oil in cold weather (from -5°C own to -40°C). This in turn translates to the resistance that the oil presents
to the moving parts of an engine during a cold startup. The CCS tries to simulate this using a cylinder with a piston-like moving part inside it that is linked to an electrical engine. The piston and cylinder are separated by few millimeters and oil is pumped into this space. The whole lot is then cooled to the temperature required to run the test and given time to reach equilibrium. The motor then tries to rotate the piston and the resistance to that rotation is measured and compared to the resistance presented by a standard oil of known resistance. According to the viscosity range, the test is carried at a defined temperature. For example, for a 25W oil, the test is carried at -10°C, for a 20W, -15°C, for a 15W, -20°C and so on.
There's a little more information on CCS devices at RandomFactsAndConjecture.com (or wikipedia as most people know it): Cold crank simulator.
Now I'm all confused. Can you simplify all that for me?
Sure. Here's a three-line table to give you an idea where typical engine oils are used:
| Oil type | Typically used in.... |
|---|---|
| 5W-30 | Cooler climates, like Sweden or Canada |
| 10W-40 | Temperate climates, like England |
| 15W-50 | Hot climates, like Italy, Spain, Egypt |