Several factors must be considered in determining the proper  grade of oil to use in a particular engine, the most important of  which are the operating load, rotational speeds, and operating  temperatures. The grade of the lubricating oil to be used  is determined by the operating conditions to be met in the  various types of engines. The oil used in aircraft reciprocating  engines has a relatively high viscosity required by: The SAE letters on an oil container are not an endorsement  or recommendation of the oil by the SAE. Although each  grade of oil is rated by an SAE number, depending on its  specific use, it may be rated with a commercial aviation  grade number or an Army and Navy specification number.  The correlation between these grade numbering systems is  shown in  Figure 6-3 .  The viscosity index is a number that indicates the effect of  temperature changes on the viscosity with the oil. When oil  has a low viscosity index, it signifies a relatively large change  of viscosity of increased temperature. The oil becomes thin  at high temperatures and thick at low temperatures. Oils with  a high viscosity index have small changes in viscosity over  a wide temperature range. The best oil for most purposes is one that maintains a constant  viscosity throughout temperature changes. Oil having a high  viscosity index resists excessive thickening when the engine  is subjected to cold temperatures. This allows for rapid  cranking speeds during starting and prompt oil circulation  during initial startup. This oil resists excessive thinning  when the engine is at operating temperature and provides  full lubrication and bearing load protection.  Generally, commercial aviation oils are classified by a  number, (such as 80, 100, 140, etc.) that is an approximation  of the viscosity as measured by a testing instrument called  the Saybolt Universal Viscosimeter. In this instrument, a  tube holds a specific quantity of the oil to be tested. The oil is  brought to an exact temperature by a liquid bath surrounding  the tube. The time in seconds required for exactly 60 cubic  centimeters of oil to flow through an accurately calibrated  orifice is recorded as a measure of the oil’s viscosity. If  actual Saybolt values were used to designate the viscosity of  oil, there would probably be several hundred grades of oil. Flash point and fire point are determined by laboratory tests  that show the temperature at which a liquid begins to give  off ignitable vapors, flash, and the temperature at which  there are sufficient vapors to support a fire. These points  are established for engine oils to determine that they can  withstand the high temperatures encountered in an engine. To simplify the selection of oils, they are often classified  under an SAE system that divides all oils into seven groups  (SAE 10 to 70) according to viscosity at either 130 °F or  210 °F. SAE ratings are purely arbitrary and bear no direct  relationship to the Saybolt or other ratings. The letter W occasionally is included in the SAE number  giving a designation, such as SAE 20W. This W indicates  that the oil, in addition to meeting the viscosity requirements  at the testing temperature specifications, is satisfactory oil  for winter use in cold climates. This should not be confused  with the W used in front of the grade or weight number that  indicates the oil is of the ashless dispersant type. Cloud point and pour point also help to indicate suitability.  The cloud point of oil is the temperature at which its wax  content, normally held in solution, begins to solidify and  separate into tiny crystals, causing the oil to appear cloudy  or hazy. The pour point of oil is the lowest temperature at  which it flows or can be poured. Although the SAE scale has eliminated some confusion in  the designation of lubricating oils, it must not be assumed  that this specification covers all the important viscosity  requirements. An SAE number indicates only the viscosity  grade or relative viscosity; it does not indicate quality or  other essential characteristics. It is well known that there are  good oils and inferior oils that have the same viscosities at a  given temperature and, therefore, are subject to classification  in the same grade. Figure 6-3.  Grade designations for aviation oils.  Commercial Aviation No. Commercial SAE No. Army and Navy  Specification No. 30 40 50 60 70 1065 1080 1100 1120 65 80 100 120 140 6-3 Cloud Point and Pour Point Flash Point and Fire Point Viscosity Index Viscosity 1.  Large engine operating clearances due to the relatively  large size of the moving parts, the different materials  used, and the different rates of expansion of the various  materials; 2.  High operating temperatures; and 3.  High bearing pressures.