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I might be wrong but oil thickness also shows its ability to handle "squashing" or somehow hold its clearance filling potential.
The thinner the oil, the worst it holds clearance between mechanical parts.
That's where you lose on reliability.
That's why it is advised not to use thin oils in older engines : oil pressure will suffer.
So putting specificated oils for your particular engine in optimum temperature viscosity wise is kind of the only reasonable way to lower its viscosity while retaining the "strength" of oil the engine has been designed with.
The main issue with playing with the limits is the ability to handle peaks.
Ever stopped your boiler as soon as water boils ?
Yes, it continues boiling despite you're not putting any more energy into it, the process has inertia.
It is the same in your engine and you can't even stop energy input at the flick of a buttton, the whole engine has heat inertia.
Basically we can play with temperature but if we get it wrong just once, the price is very high compared to the savings.
Lastly, your vehicule cooling potential is designed to handle a certain amount of kW. Bringing back some will strain and reduce that cooling potential, so you have to add some cooling potential back, adding weight and money or drastically reduce car 's flexibility.
"Can't use my car today, they forecast 70°F this afternoon ..."
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