The switch to 5w-20 was for mainly for fuel economy gains, you can run a heavier weight (within in reason) - might just see less efficiecny. Much of it is dependant on the conditions and the engine itself. Some engines like a thinner or thicker oil.
The 2ZZ-GE guys swear by 15w-40 in the engine - which I would personally feel be counter intuitive on a high revving engine, but they say their engine likes it. The difference in thickness between a 20 weight and 40 weight oil at operating temp is not grossly different, ie. a 40 weight will not be twice as "thick" as a 20 weight.
The rating is for the SAE viscosity - many attribute it to the informal notion of "thickness" - but that can be interpreted a bunch of ways, usually wrong in the case of lubricants. Best to think of it as the resistance of a fluid to deformation by an outside stress/shear.
Rotella T6 is great oil - a tad bit too thick for my needs, but an oil on my short list to run in an engine that tends to be hard on oil (ie, turbocharged, boxer engine designs, project engine, track car, etc.). Just double check how much phosphorus they use for an additive. Its an EP additive (extreme pressure) but also has a bad reputation for killing catalytic converter fast. Hence why the lastest blends of oils have pretty much little to zero phosphorus in their additive pack. Same with zinc and moly - both great for protection, but falling out in favor of more exotic additives.
