Brendan

what exactly do the numbers mean? All I know is that 20w50 is a thicker oil than 10w30. Thanks.

jimbo74

Oil weight, or viscosity, refers to how thick or thin the oil is. The temperature requirements set for oil by the Society of Automotive Engineers (SAE) is 0 degrees F (low) and 210 degrees F (high).

Oils meeting the SAE's low temperature requirements have a "W" after the viscosity rating (example: 10W), and oils that meet the high ratings have no letter (example SAE 30). An oil is rated for viscosity by heating it to a specified temperature, and then allowing it to flow out of a specifically sized hole. Its viscosity rating is determined by the length of time it takes to flow out of the hole. If it flows quickly, it gets a low rating. If it flows slowly, it gets a high rating.

Engines need oil that is thin enough for cold starts, and thick enough when the engine is hot. Since oil gets thinner when heated, and thicker when cooled, most of us use what are called multi-grade, or multi-viscosity oils. These oils meet SAE specifications for the low temperature requirements of a light oil and the high temperature requirements of a heavy oil. The "W" actually stands for Winter, not Weight as most people beleive.


here is a good read: http://www.vtr.org/maintain/oil-overview.html

Flamedx4

Oil viscosity gets thinner with higher temperature and thicker with cold. Additives - viscosity improvers - work to reduce the thickening as oil gets colder and reduce the amount of thinning as the oil heats. This feature is usually expressed as a "multi weight" oil, such as 15-40, 0-30, etc. The number means that the oil at ambient temperature has a viscosity of the lower number, and at (usually 100° centigrade the oil has the viscosity of a single grade oil at that temperature of the higher number. Other additives in motor oil include suspension additives to suspend contaminates, detergents, rust & corrsosion inhibitors, antiwear additives, anti-foamants, and pour point depressants. Neat stuff huh?

Koz

Grade/weight of oil – Viscosity (flowability) is how to measure the shear strength of a thin layer of oil. A bearing works by using a thin layer of oil not by the oil pressure generated by the pump. If you use too high a weight oil the layer will not form fully or may not get into the bearing at all. If you use too low a weight the layer will not give the bearing the proper protection. Bottom line, make sure you stay within the manufactures recommended ranges.

Multi viscosity oil is created by adding polymers (chemicals) to a base 5W, 10W, 20W. Think of a 20W – 50W as a 20 weight oil that will not thin more than a 50 weight when hot. Keep the weight range as narrow as possible. Polymers are prone to viscosity and thermal breakdown.

Viscosity Index – The Number indicating the rate of change in viscosity within a given temperature range. The higher the number, the lower the change. The higher the number the better. Important for bearing. Approx. range 110 to 210

Koz

theryanator

based on this info, what would be recommended for a 99 v6 3.4l sr5 with 134,000 on it? Synthetic or normal oil?

Should I switch oils in the winter and summer months? I live in Chicago so it can be sub-zero in the winter and 90 something in the Summer.

Sorry about the hi-jack...I have been wondering the same thing about the differences in oil...the info this site provides is invaluable...thanks!

Koz

If you don't leak oil you can use synthetic, change every 10K miles or one year, whichever comes first. Use the manufactures suggested viscosity, should be 10/40 or 5/30 (but check). It looks like you put alot of miles in a year, and the motor has some miles already, so I would use the 10/40. If it leaks, use conventional oil and change it according to the manufactures recommendation. Try to use the lower viscosity in winter and higher in summer.

Koz