HOW CAN I UNDERSTAND THE NUMBER AND LETTER SYSTEMS USED TO RATE MOTOR OILS?

In this edition, we shall consider the least understood part among the information displayed on the label of typical lube containers. It also forms the most asked questions usually from motorists (and mechanic alike) as regards the ‘number’ and ‘letter’ systems used to rate motor oils.

Most people know nothing about the number/letter systems used to rate motor oil they use in their car engine but they should! Some motorists think it’s too technical to grasps while others say it’s the responsibility of mechanics to know. But the truth is that, even the mechanics that ought to know to better, do not know any more about car than they (drivers) do.

It is your right to know whether as a motorist, mechanic or an automobile enthusiast. You owe it to yourself and your car to know just how the ‘number’ and ‘letter’ systems work.

In this article, I have provided you with an easy but simple guide to understand the number and letter systems used to rate motor oils. For every container of motor oil you see or buy from a lube shop or auto-parts shop, you will find two vital parameters printed on it, used by the lubricant industry. They are the API and SAE symbols. Both symbols say meaningfully different things about the oil, and at the same time act as a guide to selecting the right oil for your car. Therefore you should pay more attention to this article.

Firstly, the American Petroleum Institute sets the API symbol that uses the ‘letter’ system for identifying the vehicle fuel type and quality level of the motor oil. The API symbol therefore uses two letters. The ‘first letter’ indicates the vehicle fuel type for which the oil is designed. Ratings that therefore begin with an “S” are intended for petrol engines whereas ratings that begin with a “C” are meant for diesel-powered engines. The ‘second letter’ that is attached to the first letters whether “S” or “C” designates the quality level of the motor oil. Now, the higher the second letter, (which uses alphabetical letters in increasing order to demonstrate the quality level of the oil), the more advanced the oil and the more protection it offers your engine. What this implies is, for instance, an SM motor oil can be used in any petrol-powered engine where an SA, SB, SC, SD, SE, SF, SG, SH, SJ or SL oil was originally specified. And the same goes for diesel-powered engine, for example, CG-4 oil can be used in diesel fuel-powered engine where CA, CB, CC, CD, CH, CF, CF-2, or CH-4 oil has been required. It should be noted that from SA to SD and CA to CD motor oils (quality levels) are now obsolete in the international lubricant market.

Secondly, SAE symbol is the next parameter printed on the label of oil containers to be considered. The Society of Automotive Engineers sets the SAE viscosity grade. The SAE grades are usually referred to as viscosity or oil weight, that says how thick or thin an oil is. It is also a measure of oil’s thickness or resistance to flow. The SAE viscosity grade uses “number” system and sometimes with a letter ‘W’ which connotes winter. The SAE viscosity grade rates oil based upon the temperature requirements. The temperature requirement set for oil by SAE is 0 degree Fahrenheit (low) and 210 degree Fahrenheit (high). Oils meeting the SAE’s low temperature requirement have a ‘W’ symbol after the viscosity rating or number (e.g. 10W), and oils that meet the high ratings ‘no letter’ or the ‘W’ (e.g. SAE 30, SAE 40, SAE 50). What this implies is that the 10W oils will not freeze at low temperature at 0 degree Fahrenheit but will still be liquid enough for cold starts in winter. And that the SAE 30, 40, 50 oils will still be thick enough when the engine is hot in the tropical regions without necessarily loosing its lubricity. Since oils really get thinner when heated, and thicker when cooled, most of us use what are called multi-grades or multi-viscosity oils without really knowing why or the benefits for selecting so! There are two types of motor oils, thus mono-grade and multi-grade. Due to the wide variations in temperature requirements, one cannot set out to use monograde oil in winter, because it will be too thick and will not flow to engine part during cold start. But you can use multi-grade oil in both the winter season and hot weather of the tropical. Multi-grade is the answer to variations in temperature requirement occurring in oil. Multi-grade oils such as 10W 30 are designed to have the viscosity of an SAE 10W oil at cold temperatures combined with the viscosity of an SAE 30 oil at engine operating temperature (high). The “W”symbol or winter designation indicates that the oil meets viscosity requirements for low temperatures (below 30 degree Fahrenheit).

We strictly advise that when choosing an oil, that you follow the manufacturer’s recommendation (manual). We welcome your questions or comments, please send us a text or email message and we shall respond to it promptly.