By light weight I am referring to viscosity. Thinner might be a better term. The concept is that the thinner oil will get lubrication to areas measurably faster than a thicker oil and the belief is that this small amount of time (probably measured in seconds) makes a big deal. Such a deal that it becomes more important than the decrease in wear caused by higher oil pressure available with heavier oils.
The description 15-40 is a measure or statement about viscosity not its source. I say statement because the actual viscosity is a single number say 20w. The statement 15-40 infers multi-viscosity. The statement is that when cold the oil will exhibit characteristics of a 15w oil. This would be thin and a benefit to quick lubrication. The 40 refers to the statement that when hot the oil will exhibit the characteristics of a 40w oil. This will mean that it will carry more oil pressure as a heavy weight oil might. Actually a multi-viscosity oil is just the opposite. It is an oil that has its viscosity stabilized (in other words it is mostly the same viscosity all the time). So it can be thin when thin is cool and thick when thick is hot.
Synthetic means that it is created from chemicals not pulled from a mixture as is dino oil. The benefit here is that the synthetic can be designed to handle a situation whereas with dino oil you can only pick the best of what's there already. The stabilization of viscosity is one of the main design factors in synthetic oil.
Bosch Master, ASE Master, L1
33 years MB technician