Not at all. These are just grades determined by a methodology developed by the SAE. They do not represent actual weights of oil, but are in fact arbitrary numbers assigned by the SAE.
The dual grade numbers indicate that it's a multi-viscosity oil. That is, it's viscous properties (resistance to flow) vary with temperature due to the addition of long chain polymers. These polymers curl up in cold temps making the oil less viscous ('thinner'). When the oil temp rises, these polymers uncurl making the oil more viscous ('thicker'). Viscosity is a measure of how a fluid resists flowing, less viscous and it flows easier, as the viscosity increases, it is more difficult to flow - water vs honey are good examples. The 'W' stands for Winter, not weight as commonly mistaken. It indicates that at 0°F the oil will flow at the same rate as an oil the weight of the number value before the W, in a 5W40 oil, this would equate to a 5 weight oil.
The benchmark temperatures are 0°F, and 100°F. So, at 0°F (or below), a 5W40 oil will flow like a 5 weight oil, at 100°F, the oil will flow no less than a 40 weight oil. An oil with too low a viscosity can shear and lose film strength at high temperatures. An oil with too high a viscosity may not pump to the proper parts at low temperatures and the film may tear at high rpm.
Back in the day, people used to use different weight oils with different seasons (temp ranges). Multi-vis oils eliminate the need for this.
But understand that the actual base oil may not be either 5 weight or 40 weight, and usually isn't - it's usually 10 weight. It's just that the combination of polymer additives and the base oil give it the flow characteristics of the oil weights in the assigned grade - 5/40. Now, as the temp increases beyond 100°F, the oil continues to thin out. A higher graded oil, or bigger number value, will thin less than one with a smaller value, but not necessarily with any lineality.
