Sorry, but that is just not true. ALL motor oils become thinner (less viscous) with INCREASING temperature.
The multi-grade ones will not thin out AS MUCH as a single-grade but they will all become less viscous. The 5W-50 becomes as thin as a straight 50 would when hot, but it's still thinner than when it's cold.
Just google "oil viscosity chart" and you will see that every single one has a negative slope (viscosity drops with increasing temperature).