In general it is limited to certain years of engines in Porche, Saab, MB, etc. where the engine is designed to run hot and fast. Can be used in race and rally cars where although the engines are generally Honda or Toyota, the cars finish their laps running 120 degree oil temps.
5W-50 can also be used in 4x4 vehicles that go off-road (high revs, loads) and in old Mercedes cars that have working temp higher than 110 oC. Lubricants with a big viscosity differential like 5W-50 generally suffer from high temperature High Shear problems because to get the wide swing they need a lot of viscosity improver additive, which shears with time.
If Engine is of older design, it means the clearances between piston rings & cylinder liners are higher (120 micrometer) and also the Engine size. Hence to seal the blow by gases/ carbon soot from entering inside the crankcase a higher viscosity Oil preferably 5W-50 can be used, it also has a good low temperature/ cold cranking properties.
Unlikely in modern Engines the size has been reduced and clearances b/w piston rings & cylinder liners are narrowed(20 - 50 micrometer). In such a case use of higher viscosity grade would not be a wise option since it would also cause increase in internal fluid friction and reduce fuel economy. Hence optimal viscosity grades would be 5W-40, 10W-40 or 10W-30, 5W-30 etc