The 5 in 5W-40 says that the oil has the viscosity of a 5 weight oil at 100 degF. This, along with pour point, is an indicator of the oil's ability to flow and lubricate at cold startup and initial operation.
The 40 in 5W-40 says that the oil has the viscosity of a 40 weight oil at 210 degF and is one indicator of the oil's ability to lubricate at high operating temperatures.
The difference between 100 degF and 210 degF viscosity of the base oil determines the base oil's viscosity index. Depending on the base oil's viscosity index, the manufacturer/reblender will add viscosity improvers to the base oil to bring the final viscosity spread to whatever is desired for the final product.
In a conventional "dino" oil, more viscosity improver is required to achieve a wide spread in rated viscosity (35 in the case of the oil in question - 40 minus 5). There was concern about this in conventional oils because these viscosity improvers were subject to shearing and deterioration over time.
Due to the better viscosity index of synthetics, less viscosity improver is required to achieve wider viscosity spreads. Therefore, although a 5W-40 "dino" oil might be suspect for extended operation, a 5W-40 synthetic might (and probably would) be fine.
Hope this helps.
Rusty
The 40 in 5W-40 says that the oil has the viscosity of a 40 weight oil at 210 degF and is one indicator of the oil's ability to lubricate at high operating temperatures.
The difference between 100 degF and 210 degF viscosity of the base oil determines the base oil's viscosity index. Depending on the base oil's viscosity index, the manufacturer/reblender will add viscosity improvers to the base oil to bring the final viscosity spread to whatever is desired for the final product.
In a conventional "dino" oil, more viscosity improver is required to achieve a wide spread in rated viscosity (35 in the case of the oil in question - 40 minus 5). There was concern about this in conventional oils because these viscosity improvers were subject to shearing and deterioration over time.
Due to the better viscosity index of synthetics, less viscosity improver is required to achieve wider viscosity spreads. Therefore, although a 5W-40 "dino" oil might be suspect for extended operation, a 5W-40 synthetic might (and probably would) be fine.
Hope this helps.
Rusty