This is true but not entire accurate. there's more to it.

Engine builders work a lot with material types, tolerances, block inner passages, oil pump flow and pressures and so on.

They test the engines with a wide range of weight oils in different given temperatures. after this grueling test they state the weight that behaved best overall. And the sometimes.es in owners manuals states also extreme condition recommendations.

Is not wise to your an 0W30 if the recommended weight is 15w30 and your cruising in Irak . based on the fact that thinner oil flows better.

That oil weight will not have the capacity to make the cushion between metals and won't maintain the pressures required.

Is an example.

Choose wise your oil weight and among all base your decision on manufacture and weather conditions.

Just my 2 cents .