Motor oil is classified by an outfit called the SAE which means Society of Automotive Engineers. This is one of the largest and oldest technical societies in the world and they have a lot to do with standards for all kinds of things both related to and not related to cars. They are very active in the aerospace field as well, for example. I have a set of their handbooks in my office which describes the standards and it is 4 thick volumes. The section on oils and lubricants is several hundred pages long - so these folks know about oil.
One of the everyday things that the SAE writes standards on is indeed, motor oil.
When you buy a can of oil look for a round symbol which will say something like: "SAE 10W30" inside the inner circle and "API SERVICE SG" or SF or CD or CE or something like that above the horizontal line in the outer circle.
Below the line in the outer circle is a space for the words "Energy Conserving" which can only be present if that oil has been proven to lower fuel consumption by something like 1.5% in carefully done tests.
The SAE 10W30 is the viscosity of the oil. Viscosity is basically how thick or gooey the oil is and it is determined in a test using very specific procedures laid down by the American Society for Testing and Materials (engineers love technical societies). Basically, the higher the number - the thicker the oil at a given temperature. Note that the oil thins or becomes less visous at higher temperatures. Don't worry what the units are - it doesn't matter for our purposes.
The fact that there are two numbers means that the oil has been formulated to have two viscosity characteristics. The first number (the "10") means that the oil follows the viscosity curve of 10 wieght oil at lower temperatures and the "30" means that it follows the curve of 30 wieght oil at high temperatures. It does NOT mean that the oil thickens as it get hotter - it just means that it doesn't get as thin as quickly as the temperature increases.
Now why do we want oil to be thicker or thinner?
| we want thinner oil to ensure that the oil can circulate through the entire lubrication system quickly at start-up and also to decrease fluid friction which eats up horsepower. |
| we want thicker oil to ensure that the oil coats all of the engine parts during engine off times so that the bearings are not dry when the engine first turns over but before oil pressure is established by the oil pump (which is of course dirven by the crank or camshaft). A thicker oil will also bear a larger load ensuring that no metal-to-metal (ouch!) contact occurs in a bearing. |
So you see that these two characteristics are basically in conflict with each other - thicker means better lubrication on start-up if the oil is already in place in the bearing and better lubrication at high temps, and thinner means better lubrication on start-up if the bearings are dry and lower friction at higher engine speeds.
This is the basic reason for the multi-viscosity oil - better cold starts and better lube at high temps. The dry bearing problem is looked after by careful bearing design to always ensure that there is some oil left in a bearing when it is shut down to protect it when the engine is started.
What about the API thing?
The API (American Petroleum Institute) who define the service grade which is the little two letter code above the horizontal line in the circular symbol. The API service grade is basically a guide as to what type of engine should use that particular oil. Modern gasoline engines use Sx service grades where the x is as high a letter as you can find - I think that latest current service grade letter is a G so the most "modern" oil is service grade SG. [It's SH now!] For diesels the service grades are Cx and the current one is CE, I think (but I don't have any diesels so I am not sure).
Do not use oils for diesel service in gasoline engines or vice versa - they are not correctly formulated and will not work properly.
Points to Note:
The G in the service grade is not like a shelf life "best before" date. It just means that the API has come out with an up-graded standard and the new oils must meet it in order to be labelled with the designation. You may find some SF or SE grade oil on store shelves and it isn't necessarily bad to use it - when our Fieros were new (7-11 years ago) the current service grade then was SF I believe.
The oils are downwardly compatible - you can use SG oil in an old car with no problems but I wouldn't use SD or SE oil in a newer car (I doubt that you could even find a bottle of it now anyway).
The whole point of this is that between the SAE viscosity standard and the API service grade designation you can't go wrong on oil since any oil bearing these symbols has met rigorous standards. The only problem could be if you got some oil from a crooked company who labelled their product without complying with the tests or if you used the wrong viscosity oil.
Do not go to a higher visosity grade unless you need it for very high temp operation (like in the desert) or to help with an oil consumption problem (going to a more viscous oil can decrease leakage at worn seals to a small extent - but don't count on it working for long). If you get the viscosity too high you will actually cause more wear in the enigne since you will be delaying circulation on start-up and you will be wasting fuel.
From: Dr. Peter Frise
