What is monograde and multigrade lubricant?



Monograde oil (ie. SAE 40) behaves in a linear relationship to temperature. The higher the temperature, the thinner the oil gets and the lower the temperature, the thicker the oil gets.

Because of this characteristic of engine oil, if we are using monograde oil, we have to change to a thicker oil (ie. SAE 40 or 50) during the summer and change back to thinner oil (ie. SAE 30) during winter.

SAE index may have a letter W (such as SAE 20W) which indicates the oils suitability for use in colder climate.

Technology advancement and we now have additives that will help reduce the change in engine oil thickness due to temperature. These are called multigrade oils which behave like a thicker oil when the engine is hot (running) and behave like a thinner oil when the engine is cold (starting up). With the arrival of multigrade oils, we no longer require to change to different thickness/viscosity of oil depending on the weather season. We can now use one multigrade oil for all seasons. The multigrade oil with SAE index number being SAE 20W/50 indicates thicknesses at low and high temperature respectively.

From EMAT Oil, Mydreamoil.com

ECONOMECHANIX
4404 NW 13th Street #25, Gainesville, Florida 32609
(352) 376-5624 / Fax (352) 376-5624

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The most important system to the "health" of your engine is the lubication system. Oils have changed a lot in the past few years, and a lot of confusing information is out there. Hopefully this will help.
OIL VISCOSITY
An oil's viscosity is just how thick it is. Viscosity is measured by an oil's "weight" Way back when all cars used 30 wt (weight) oil. This is a single grade oil: its actual viscosity varies with temperature. single grade oil gets thinner when hot and thicker when cold. Multigrade oils have labels like "10w-40" or "20w-50". Their thickness changes with temperature also, but they change the opposite way from single grade oils. A 10w-40 motor oil behaves like a 10 wt oil when cold and behaves like a 40 wt oil when hot.

Generally a thicker oil will withstand more abuse, however thick oils can take longer to reach engine parts when the engine is started. In cold climates a thinner oil (lower "wt" number) is often used to compensate for this. Engine startup is when most engine wear occurs: it runs for few seconds without oil pressure every time you start it.

Manufacturers are using thinner and thinner oils in cars. (5w-30 wt is recommended for many new cars.) This is primarily for fuel efficiency. In Florida I've never seen a car have problems from using a thicker oil than this, but then again I haven't seen any of the new ones burn up engines from these thinner oils. I have seen higher mileage engines consume more oil of the lower viscosity types: if your engine uses more than 1 qt every 1000 miles you might use a higher viscosity than recommended by the manufacturer. Another reason to go with a slightly higher viscosity would be if you anticipate driving in extreme high temperatures. Otherwise, I'd stick with whatever grade the manufacturer recommends.



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Q. WHAT IS A MULTIGRADE ENGINE OIL ?

A. Multigrade engine oils are formulated to meet the requirements of more than one SAE viscosity grade classification, and therefore may be used over a wider temperature range than single grade oils. A multigrade oil is identified by two SAE grade designations. For example, an SAE 10W-30 designation indicates that the oil acts like a 10W oil at cold temperatures and a 30 grade oil at normal operating temperatures.

Taken from http://www.lubeoils.com/generalfaq.html#38m