Lighter pistons produce less inertia and move around less in the bore which helps ring seal. Lighter pistons, rods, and rings means less bobweight at the crank which means smaller counterweights, reducing windage. This is really only a concern though at high rpm (>7k rpm).
It's interesting though that German automotive engineers never pushed for light motor oils. German cars had a good reputation, especially in the 80's and 90's. Even now they're good vehicles, it's just that for one, they have a lot of things that you as an owner need to keep track of, and the other thing is that they are built to be maintained. Because if you don't, and most owners don't, then they go to waste.
Transmissions are a different story altogether. For transmissions, thin ATF fluid seems to work well as long as the transmission is well designed. But things are different in engines. You have forces pushing and pulling, rotational mass, pistons changing direction up and down, timing chains, and so on. In an engine, the oil has to deal with way more than in a transmission, including contamination from fuel and combustion air (particles that get in). For example, a thicker oil film will hold particles that otherwise cause wear much better than a thin oil.
What gets me is how LSPI has become such a huge justification to reformulate all mainstream motor oils. Wouldn't it have been easier to have a separate lineup, lets say of 0W-20 oils (since GM loves them so much) formulated just to deal with LSPI issues? It's a bandaid solution anyway, and not even a good one. Seized turbos and blown motors are nothing new when it comes to those anemic 1.5L and smaller engines. I rented an Opel Insignia last year in Europe with a 1.5L Turbo. It was gutless and no fun to drive. At least I rented a 6 speed manual. It was weak, and at best, it had the fuel economy of a 2.4L GDI engine (if I was babying it).
Anyway, I am starting to wonder about this ever greater push for thinner oils. Most engines I owned seemed happy with an oil that is 9 to 12 cSt at 100C. Why the push for thinner and thinner oil? Might as well build engines with polymer-coated bearings and moly coated pistons and run straight 0W in them, as ~3 cSt @ 100C oughta do it. Okay, my small rant is over.
