[Razorback1980]

Read the data sheets on oils and there is a lot of information that you can quickly learn about each of the oil brands and some of the qualities. Castrol sponsored my race cars for years, so hanging around oil engineers, I can repeat what Pete has told me.

There are really six different properties for oil that you should pay attention to...viscosity, viscosity index, flash point, pour point, % sulfated ash, and % zinc.

Viscosity is the measure of how well an oil flows...I bet each of you knew that didn't you? Actually it's the property of an oil to develop and maintain a shearing stress dependent on flow and then to offer continued resistance to flow. Thicker oils generally have a higher viscosity and thinner oils a lower. The shear stress is what provides lubrication for the engine. Too thick an oil and the film will not form evenly if it gets in the gaps at all...also too thick an oil film and it will tear at high rpms....too thin and the shear stress of the film will be so low that the bearings and their shafts will rub on the high places. These numbers are derived by measuring the oil flow at specific temperatures. The low number is the viscosity of the oil at 0 F. The W tells you that the oil meets certain specs at 0 and therefore approved for use at lower temperatures.

Multi-viscosity oils are engineered to work the opposite of normal fluids with the exception of water. Oil companies add polymers to the oil to get them to be thinner at cold temperatures and thicker at higher temperatures. That sounds like a very good thing but there are draw backs to having those properties. The wider the range of the lower and upper number, the more polymers they have to add to the oil. Polymers are great to changing the flow of the oil but have no lubricating properties to them. In other words 20W50 oil is 20 weight oil that will not thin more than 50 weight when hot. While adding those polymers was one of the best things to happen to oil, it's best to choose wisely and use them to your best advantage. In other words ALWAYS choose the oil with the narrowest span of vicosity numbers that will operate in the temperatures you think your car will be in. Lubrication isn't the only problem associated with having all those polymers. The higher the polymer, the higher the sludge deposits and increased problems such as collapsed lifters and sticky rings. Bottom line is fewer polymers = better performing engine with less oil related problems. Basically a 10W40 oil has less lubricating efficiencies than a straight 30W oil. In fact, most manufactures will void warranties if you use a 10W40 oil any more. With that said, I run a 20W50 oil in my car which has the same point spread that a 10W40 oil does, but there is one major difference...it starts with a heavier base so it requires less polymers to provide the change in viscosity...because it has less polymers and more oil to form the film, it lubricates better and is not as subject to viscosity and thermal breakdown. Oils such as Mobil 1 5W30 oil have no viscosity improvers in their oil which is why it works so well and so many auto manufacturers use that oil. Certain weights of AMSOil also use no polymers as I'm sure there are a few other manufacturers have oils blended like that but I'm unsure what they all are.

Viscosity index is the rate of change an oil changes viscosity within a temperature range. This number describes the amount of lubrication and is what keeps bearings lubricated.

Flash points are another indicator of how good an oil is. Flash points are the temps that oil gives off a vapor that can be ignited. The lower the flash point, the more it will burn off cylinder walls which run at a very high temperature. If it burns off, there is no lubrication. The higher the flash point, the better the oil and the higher the grade of crude oil used to manufacture the oil.

Pour Point is a number of very high importance although it's seldom looked at. Pour point is the lowest temperature that an oil will pump and maintain adequate oil pressure. It's derived by measuring the temperature that a chilled oil shows no signs of movement on a slanted surface plus 5 degrees.

% sulfated ash. This measurement is the amount of deposits left behind after oil burns. The lower the sulfated ash, the less deposits you will have in your engine.

% zinc. This is the amount of zinc added to the oil as an extreme pressure additive. Under normal operations, zinc doesn't really do much for an engine. It only helps when an actual metal part rubs on another metal part. The zinc reacts with the metal to prevent wear and damage. Zinc is especially useful in a high revving or turbo engine. The zinc doesn't give more protection, it provides a longer protection. Of course with it being an additive, it will also lead to more deposits and other engine problems.