Here’s the straight story on viscosity grade. The “W” number in, for example, a 15W-50 motor oil outlines the viscosity requirements of the oil when it is cold. This means that a 15W-rated oil has to meet certain criteria at cold temperatures. Taking our 50-weight example, a 15W-rated oil will flow more freely than a 20W-rated oil when compared at cold temperatures.
Now let’s talk about the “50” portion of a 15W-50, 20W-50 and straight SAE 50. The Society of Automotive Engineers (SAE) states that a 50-weight oil must have a viscosity at 100°C above 16.3 cSt but less than 21.9 cSt and have a high-shear-rate viscosity minimum of 3.7 at 150°C. This means that a 15W-50, 20W50 and SAE 50 weight must all meet these requirements. This does not mean they respond the same but they all need to meet these requirements regardless to have the 50-weight designation.
Against this backdrop of understanding, the question is, should you run a multi-weight or a straight weight in our example? The answer is, take a close look at the oil specifications and compare to what you want to do and pick an oil that fully meets your needs. The best question to ask is, how does the oil hold up under shear”? Shearing is the loss of oil viscosity during operation. Shear is especially a problem when running engines at high rpms.
Typically, muscle car folks will have a tendency to run up the rpm from time to time, which can shear down the oil viscosity. Viscosity is one of the most important factors for overall engine protection. When oil viscosity at operating temperatures degrades significantly, engine protection will be compromised. On the other side of the coin, if you start and run your muscle car in colder climates, a multi-weight will definitely provide more oil flow to critical parts quicker. Remember, most engine wear on automobiles actually occurs on startup when engine oil is not flowing readily to engine components.
Viscosity selection tip: pick a multi-weight oil that has outstanding shear stability. I personally always recommend AMSOIL.