I challenge the next person who says "optimal" wrt viscosity, to qualify it.
What are you talking about? What's not optimal? Wear protection? Fuel consumption? Is it your prime concern to ensure that extra tenth of a MPG in the first minute of driving? It is for automakers, and their CAFE, but is that what you yourself are trying to optimize? How about flow? Is it flow? What are the consequences of flow speeds on a warming engine?
Oil X isn't the optimal viscosity. Automobiles are not the optimal mode of personal transportation. The ICE is not the optimal engine for the not optimal automobile running a non optimal viscosity. Any and everything is not optimal in some regard; we absolutely have to qualify it though, for it to mean something other than an abstraction.
As usual the viscosity issue takes the form of religious jihad; fears are being manufactured, and subsequently blown way out of proportion, supported by nothing but imaginations.
And no end is in sight. Tomorrow, there will be another thread "does 0w20 really protect anything?" and "does anyone still use motor oil grades from the cretaceous period? so old, must be horrible, made for pterydactyls" etc etc
It's one thing to stay on top of tribological evolutions, and equally critical to be aware of the driving forces and intentions behind the chemistries. Base stocks and the blends thereof have skyrocketed in oxidation stability, purity (or lack of undesirable contaminants if "purity" sounds too hokey for you) VI, and volatility. PMA polymeric VIIs have become further optimized for stability at common engine temps by molecular shape and length. But all that means in today's common formula is that they can push the boundaries of viscosity even further, optimizing them for VI/(CA)FE performance, while still maintaining the same or slightly improved stability of the outgoing product.
That's cool.
Except I don't care about CAFE. I don't want those boundaries pushed. I don't care about fractions of a percent increase in fuel economy/reduction of drag for the first ~4 minutes of engine operation. I really don't. They're not going to give me an extra 5MPG per tank.
What I do care about is benefiting from skyrocketed quality of the new bases, without re-trading some of that cleanliness and stability back for more VI, when I don't need it.
And I'm thankful that this can be had, by choosing grades of oil made with these new fantastic base oil blends, that haven't been pushed right to the edge of VI/FE performance. I'm talking about the 5w20s, 10w30s, 15w40s and straight grades made with GrIII+ ... and sure if i was taking a road trip, you bet your balls I'd consider buying a jug of quality GrIII+/IV/V SAE20 or SAE30 (AMSOIL for example)- as long as it's made with the same bases that carry the same uprated stability as it's peers within a product line. I like margins of protection and free moving piston rings.
If a 0w20 product of the same line uses even more stable base stocks (to make it perform suitably/per manufacturers spec) then I would gladly pick that product up- and because it would be needed (cold season) but not VI for VI's sake. I always maintain that there is no free lunch with motor oil. There is no motor oil that will satisfy CAFE and a brutal track day and do both optimally. But those are the demands I put on MY car. Maybe you drive much more sedately and couldn't care less/don't know/care about ring pack coking, volatility consumption, temporary/permanent shear-- but I do and passionately so, and will continue to investigate each oil's formulation to find the best one for the benefits that I want. Specs won't tell me that, that's why I'm here!