Choosing the correct oil viscosity can be an important factor in determining the life expectancy and performance of your engine. Using the correct-viscosity oil can also keep the engine running at its peak efficiency, and plays a small part in the overall fuel economy of your vehicle. Modern engines have very different viscosity requirements over vintage engines, and manufacturers invest a lot of time and money to determine which mixture of oil and oil additives will work best with their engine components. This is why you often see original equipment recommendations for new weight ranges that were unheard of a few decades ago. These new formulations will eventually lead to the replacement of non-detergent straight-weight oils as well as some of the old, reliable multi-viscosity oils of the last five decades.
Simply stated, viscosity is merely a measurement of oil and other fluids’ resistance to flow. This flow can best be described as the difference between pouring water (with a viscosity of 1) through a funnel and then molasses (which has a viscosity of about 2,000) through the same funnel. The amount of friction from the contents of the molasses causes it to flow at a much slower rate than the water. Viscosity measures the amount of friction that is within the oil, preventing it from moving freely. Higher-viscosity oils have more friction and flow more slowly than a low-viscosity fluid. This friction is caused by the makeup of the molecules used to create the oil byproduct during manufacture. Polymers are added to the mineral oil base to reduce changes in viscosity when the oil is subjected to extreme temperatures at either end of their effectiveness. There is a limit to just how much polymer material can be added, though: While heavier polymers are good for thickening oil for a wider range of temperatures, they also have a lower resistance to mechanical shear than lighter polymers or base mineral oil, so it is a complicated balance that is sought when engine oil is formulated.
Viscosity is measured in several different ways and is numbered using industry-standard scales. Engine oils are measured on an SAE crankcase scale, while hydraulic fluids are measured on an ISO VG scale. Gear oils also use their own SAE gear scale and tractor/industrial fluids use an AGMA scale. Both SAE scales measure kinematic viscosity at 100 degrees Fahrenheit, while ISO and AGMA-rated oils measure kinematic viscosity at 40 degrees Celsius (104 degrees Fahrenheit). Consequently, you will find that oils from all four ratings systems will have the same viscosity (friction resistance), but are numbered on a totally different number scale. For instance, ISO 32 hydraulic fluid, commonly used in snow plows, jacks, tractors and construction equipment, has the same viscosity as 15W engine oil and 75W gear oil, but is too light to be recognized on the AGMA scale. Grade 5 AGMA-rated oil has the same viscosity as ISO 220, 50 weight SAE engine oil and 90W SAE gear oil. This does not mean that you can use ISO 32 in your rear end or 90W gear oil in your crankcase, only that the viscosities are similar. The base oil product and additives mixed into it determine the scale on which the oil should be rated. When ISO and AGMA oils can be interchanged, both ratings will be listed in the owner’s manual.
When comparing multi-grade oils, the lower number represents the oil’s viscosity at the lowest recommended temperature, the higher number representing its viscosity at the upper end of its recommended operating temperature. For example, 10W-40 is recommended for ambient (outside air) temperatures of +5 F to +122 F, while 5W-30 works best in temperatures between -22 F and +86 F. Good old 30W straight oil is currently only rated for +32 F to +86 F, and even 20W is not rated for temperatures below 14 F.
When selecting the oil that best suits your particular vehicle, your best information can be found in the owner’s manual. The factory’s recommendations will be listed there and you will usually be given a few alternatives, based on your local climate and driving habits. It should be noted that many manufacturers also recommend that you change the viscosity of your oil at least once a year based on local temperature changes. If you do not have a manual, follow the temperature range recommendations listed on an SAE engine oil chart or locate a Chek-Chart Classic Classification Guide from Motor Information Systems (Motor Manuals). Several are available, including one for 1950-1989 vehicles. Valvoline and other manufacturers also produce Lubrication Recommendation and Capacities catalogs every five years or so with updated information for engine, drivetrain and differential requirements as well as oil recommendations for small engines, tractors, outboards, motorcycles and heavy-duty trucks.