One more point is that it appears to me that most of us have no idea what viscosity is. So for those that fall into that category, viscosity is the thickness of the oil and is measured by the time it takes to flow through a hole. Now that sounds simple, but it is not the full story. Effectively viscosity relates to the molecular length of the hydrocarbon chain, but because the test is so simple, it does not differentiate between blended multi length chains and oils having chains all the same length, as both the blended and unblended chains both pass the same viscosity test. That is very important to know because blended (Multi-grade) oils have very poor sheer strength in comparison to straight grade oils. The advantage of blended oils is the viscosity index number which reflects the amount of viscosity change over temperature change. In our case where our machines operate at room temperature, a low index number is an advantage. In other words, we should never use blended multi-grade oils. We need to have maximum sheer strength. Simply because we don't need multi-temperature ability.
Of course viscosity is still not the whole story either. I will briefly talk about performance and the real difference between the different types of oil. Oil companies add a chemical package to the base oils to enhance the oils performance in their intended application role. These packages are secret and are rarely ever stated, but they enhance or attenuate certain attributes, like extreme pressure lubricity (adding metallic powders), chemicals that enhance particulate suspension or coagulation where contaminating particulate coagulates and settles out of the oil and settling to the bottom of the sump. This is why we should never use SAE automotive oils in our machines, we do not want to keep foreign particulate in suspension because we don't have oil filters that are designed to filter the contaminates.
I am not an oil specialist by any means, just a curious consumer that has found this information almost non-existent and very difficult to find. I have tried to put this info in layman terms. Please don't shoot me if I am incorrect. This is simply how I understand the subject.