Let me give some clarification on oil viscosity and weight ratings:
Viscosity is expressed in terms of the time required for a standard quantity of the fluid (at a certain temperature) to flow through a standard sized orifice, now commonly reported in centistokes (cSt), measured at either 40°C or 100 °C (ASTM Method D445 - Kinematic Viscosity).
The higher the value, the more viscous the fluid. Since viscosity varies inversely with temperature, its value is meaningless unless accompanied by the temperature at which it is determined. All oils, regardless of weight or multi-vis weight rating, will always have a lower centistoke measurement (viscosity) as temperature increases.
Below are the approximate centistoke ranges for various SAE "straight weight" oils measured at 40°C and 100 °C
Weight 40°C 100 °C
5 18-20 3.8-4.0
10 20-35 4.0-5.5
20 35-78 5.5-9.5
30 78-120 9.5-12.5
40 120-170 12.5-16.5
50 170-270 16.5-22
So an oil that is rated 20W50 would be expected to flow in the 20 weight viscosity range at 40°C (35-78 cSt) and the 50 weight oil range at 100 °C (16.5-22 cSt). The oil is still considerably thinner at the higher temperature, just not as thin as a straight 20 weight oil would be.
The SAE cold weather oil rating (the first number in the SAE multi-vis classification) actually gets considerably more complicated than this, and is not actually determined based on the oil's viscosity rating at low temps, but rather on some other cold weather type measurements of cranking rate and pumping rate, but this is a good way to think about multi-viscosity oils purely from viscosity vs temperature standpoint.