I just bought a 10w-50 motor oil!

When selecting oil for automobile engine and with keeping in mind that a good engine oil grade should be able to:
a) Flow and lubricate during cold start, in the form of liquid that flows easily at low or lowest temperature
b) Withstand highest heat and stress in engine, at hottest possible temperature without thining out (when engine oil lost its lubracating capability)

In general, the synthetic multigrade oil is the chosen one. And, why motor oil for racing use is monograde or monograde?

Theoretically, supposely ideal motor oils should have the lowest 'winter number' (a) and highest upper number (b), i.e: (a)w - (b). The oil grade that extend from 0w-60 or 10w-50, compares to what usually available like 10w-30, 15w-40 or 20w-40.

But, why so people says it is better to get a 10w-30 or 20w-40 is better than 10w-50 instead?
The reason: For oils at the same par, the 'working' range for a 10w-50 is far more than a 10w-30 or 20w-40.
A 10w-50 ranges from 10 to 50, 50-10=40;
A 15w-40 ranges from 15 to 40, 40-15=25; and
A 20w-40 ranges from 20 to 40, 40-20=20.

As you might aware, to extend the capability (viscosity index) of a motor oil to behave like a funtional oil additives like VII (viscosity index improver) and pour-point enhancer are added to based-stock. These enable manipulation of its chracteristics, also means the more the range in grade the more additives are added.

One of the shortcoming is the more the content of additive the less stable the oil compares to its monograde counterpart or based-stock. Why? The additives tend to disintegrate by shear, stress and intense heat.

I am NOT saying any oil that has wider grade range is inferior, in fact they are the greatest choice as all season motor oil, a SAFE BET. To generalise, most 10w-50 oil are among the high specification oil mostly only achievable by synthetic oil and alot of high tech R&D was done to accomplished it.

But they might be a bit costly for the extra (patented aka expensive additives) and that you can get from narrow if you know what your environment temperature, also the working temperature and load or your car engine. Possibly, you only be needing 10w-30 oil for your car or may be a 20w-40.