So what do the numbers on the motor oil mean?

1. The API service rating – this is a two letter rating that basically tells you the type of engine the oil is for ie gas or diesel and the quality level

2. The  next is viscosity grade for example 5W-30. This is the thickness of the oil. Water has a low viscosity and something like molasses would have a very high viscosity.

3. The last bit of information may be there or not bus basically tell you whether or not it’s “energy conserving”

Viscosity is ordinarily expressed in terms of the time required for a st­andard quantity of the fluid at a certain temperature to flow through a standard orifice. The higher the value, the more viscous the fluid. Since viscosity varies inversely with temperature, its value is meaningless unless accompanied by the temperature at which it is determined. With petroleum oils, viscosity is now commonly reported in centistokes (cSt), measured at either 40°C or 100 °C (ASTM Method D445 – Kinematic Viscosity).

a SAE 30 motor oil is the same viscosity as a 10w-30 or 5W-30 at 210° (100° C). The difference is when the viscosity is tested at a much colder temperature. For example, a 5W-30 motor oil performs like a SAE 5 motor oil would perform at the cold temperature specified, but still has the SAE 30 viscosity at 210° F (100° C) which is engine operating temperature. This allows the engine to get quick oil flow when it is started cold verses dry running until lubricant either warms up sufficiently or is finally forced through the engine oil system. The advantages of a low W viscosity number is obvious. The quicker the oil flows cold, the less dry running. Less dry running means much less engine wear.

Multi-weight oils (such as 10W-30) are  made possible by adding polymers to oil. The polymers allow the oil to have different weights at different temperatures. The first number indicates the viscosity of the oil at a cold temperature, while the second number indicates the viscosity at operating temperature. Another way of looking at multi-vis oils is to think of a 20W-50 as a 20 weight oil that will not thin more than a 50 weight would when hot.

You may hear people say that they wouldn’t use a 5W-30 motor oil because it is, “Too thin.” Then they may use a 10W-30 or SAE 30 motor oil. At engine operating temperatures these oils are the same. The only time the 5W-30 oil is “thin” is at cold start up conditions where you need it to be “thin.”