Masonry Magazine September 1969 Page. 16
If you're confused in buying engine oil... join the crowd
By Melvin E. Long
Choosing the best oil for an engine is essential if you want long, dependable service from a machine. But such obvious indicators as color, feel or odor are worthless. And such terms as premium, heavy duty, detergent and multiple viscosity often only increase the confusion.
The most noticeable characteristic of an oil is its thickness of viscosity. The SAE number for different weights of oil is based on its viscosity. The full official range of these numbers is: 5W, 10W, 20W, 20, 30, 40, and 50.
This viscosity is determined by measuring the number of seconds required for a certain amount of oil to pour through a small hole. The oils with W after the number are intended for cold-weather use while the regular oils are for hot-weather use.
Multiple viscosity oils, such as SAE 5W20 or 10W30, have the low-temperature characteristic of 5W or 10W, but act like 20 or 30 when the engine reaches operating temperature.
There are several properties of oils by which they may be classified. The most logical way of describing an oil is to indicate the type of job it is best suited to perform. To provide a uniform system of describing oils, the American Petroleum Institute has set up API standards. These are divided into two general categories-spark (M) and diesel (D).
1 "For Service ML" for spark-ignition engines in light and favorable service. Examples: Light trucks, some tractors, and power units.
2 "For Service MM" for controlling deposits or bearing corrosion when crankshaft temperatures are high. Examples: Industrial tractors, truck and automobiles.
3 "For Service MS" for controlling deposits or bearing corrosion because of operating conditions, fuel or engine-design characteristics. Examples: Commercial trucks making long trips or hauling heavy loads, autos used for start and stop driving at low-engine temperature and industrial tractors doing winter work.
4 "For Service DG" for diesel engines with no severe requirements. Examples: Some wheel and crawler tractors, trenching machines and stationary engines.
5 "For Service DM" for moderately high operating temperatures and loading. Examples: Some wheel and crawler tractors.
6 "For Service DS" for high operating temperature or severe loading. Used in engines using fuels or having design characteristics that tend to produce deposits or abnormal wear. Also used if conditions of service are so severe that DG class oils will not serve adequately.
Refiners can classify their oil as suitable for more than one service condition-for example, "For Service DG and MS." This oil is suitable for less severe service conditions in diesel engines, as well as the most-severe conditions in gasoline engines.
To perform all the jobs expected of it, engine oil contains many additives, in addition to the base petroleum stock.
Oxidation inhibitor additives reduce rate of oxidation and slow down formation of compounds harmful to the engine. Corrosion inhibitors coat metal surfaces with an impervious film to reduce attacks by corrosive compounds.
Detergents loosen deposits which may have formed on valves, pistons, rings, and other parts of the engine. Dispersants hold the contaminants in suspension so they will not settle out and collect in the engine.
Anti-foamants prevent accumulation of air bubbles. When oil is agitated in the presence of air, tiny bubbles form which reduce effectiveness of the oil. Therefore, these bubbles must be able to escape rapidly.
Pour-point depressants improve ability of the oil to flow at low temperature.
These highly compound, additive-type oils are not necessarily the best choice for all engines. Engines built before these additive oils were in general use were not designed to make use of the improved oil properties. Thus, there is little reason to use anything but straight petroleum oil for which the engine was designed.
Attempting to stretch oil service life by a few more hours or a few more miles is usually false economy. One engine over-haul that could have been avoided will buy a lot of oil.