Why The Obsession With 20W50 Engine Oil?

Virtually every mechanic recommends the use of 20w50 esp Mobil XHP.

This is despite the recommended 5W30/5w20 for most millennium cars (toyotas).

They say the reason is that once the mileage is above 100,000 it is better to use the heavier 20W50.

Please is this true?

What oil should be used in a 1999 toyota camry with 120k,2006 toyota sienna with 100k and 2006 toyota corolla with 90k miles.