The reason why 10w-30 and 15w-40 and 20-50 were all popular and are still available today Is because Back in the day they were more shear stable, they used group VI modifiers that would break down and the more there was in the oil the more it would shear. The larger the viscosity rating (0w-40 Vs. 15w-40) the more the oil would become thinner with use.
Now today The VI modifiers are much better and don't break down, so now the VI modifiers are wanted because the advantage of having a thinner oil at start up is much greater now that the risk oil sharing is gone.
we are now in an age of 0w oils or 5w-50s instead of super thick 20w-50s and 15w-40s.
With synthetic oil that shear stability is ever better because unlike Dinos that use a 10w base to start with then uses those modifiers to turn into a 30w at operating temps synthetic uses a 30w to begin with then they formulate it to resist thickening up when it cools down.
Thats my understanding of it but if you want a 100% accurate story on Visc then check out Bobistheoilguy and learn from the very many knowledgeable members there.
2001 TJ 4.0L Automatic, D35/30, both hard and soft top, full doors,JKS quicker Disconnects, 31" discoverer ATR, American racing 767 offset 15" wheel. ProComp Genuine steel bumpers front and back. glass pack muffler, Rugged Ridge extended fender flares, 130w KC daylighters, motorcycle light for reverse lights. 86,000k miles (11-7-2012)