The external temperature relation is valid, but its for cold starting. A 20w50 oil will be thick like refrigerated honey at -10c and the oil pump will be starved on startup till some eventual internal friction happens allowing the oil to somewhat move.
This is why the number before the w matters.
The number after the w is what matters when the engine is at operating temperature. The engine bearings and accessory parts like vvt require a certain flow and pressure balance to keep everything lubed and “cool” also.
This is why its best to follow the engine manufacturer advice in choosing best rating of engine oil forecasting how low the future weather could be. In a nutshell 0w20 is perfect for all seasons, no matter if you are in ksa, gobi desert or siberia.
Using a heavy oil just because ustaad said so is a bad idea as a thicker oil would open the oil pump relief bypass quickly and oil would not flow in required volume to cool and clean the engine parts. You will end up with a sludgy and worn engine.
