I have often heard that high viscosity oil (higher than what the manufacturer recommends) is good for old engines, engines that are nearing the end of useful life.
So, for example let's consider a 1989 Opel Vectra (a car I used to have). The manufacture recommended 5W-30 for this particular car. However, when nearing the end of its useful life, it's possible to use a higher viscosity oil such as 5W-50 (I'm not sure if 5W-50 was available in 1989, as the difference between 5 and 50 is great). It should have the same cold start viscosity, yet the viscosity in operating temperature is higher.
Is there any benefit in doing this? I think that because an old engine may have worn parts having bigger gaps, a higher viscosity could be beneficial in protecting the engine. But is it really?
Let's limit the viscosity changes only to operating temperature viscosity changes. Of course you'll run into problems if using SAE 50 in a cold winter.