I'm not sure, but all of this is really quite confusing to me...
But generally, I always noticed that 20W - 50 is always the (cheaper) "mineral" based engine-oil that is always mentioned to be (too thick) and highly-recommended to be used in older car engines...
0W - 50 on the other hand is always the (more expensive) "fully-synthetic" engine-oil that is only recommended to be used in brand-new car engines... (because it is too thin to be used in older car engines...)
How true is that?
In addition, I have also never ever came across a 20W - 50 engine-oil that is "fully-synthetic" before... (Why isn't there any out there?)
And I have also never ever came across a 0W - 50 " engine-oil that is "mineral" based before... (Why is that so?)
I really don't know, but going for a lower second number such as the xW - 30 (like you said) will mean that my engine will have lesser protection at higher-temperatures..., as well as the engine-oil will also "break-down" faster and hence; need to be replaced faster (because of lower endurance/resilience to heat)... (Isn't it?)
But based on your information/theory; the 0W - 50 grade will be one heck of a super "thick" engine-oil isn't it? (I thought that the racing industry already no longer uses thick/super-thick engine-oils for their race vehicle's engines anymore...
) - I was informed that now a days all racing vehicles are using much thinner fully-synthetic based engine-oils (for a higher-performance), such as the 0W - 50/0W - 60 grade, that comes with the same high resistance/endurance to heat & sheer at high temperatures...
For example;
According to my understanding, an engine-oil with a grade of 0W -50 means that this particular oil is thin (super-thin), it flows very easily, but yet it also has the incredibly high resistance to heat & sheer at higher-temperatures; it will not break down easily, and is very ideal to be used in brand new high-performance racing engines...
This is what I understand, though I am still looking forward to the correct answer/information (if mine is really incorrect). I'm also definitely in to learn more.
The Petronas Syntium 1000 is a Fully Synthetic 15w50 engine oil.