Essentially this sounds like an urban myth. Old car engines are meant to be improved by using thicker oil (e.g 20/50 instead of 10/30). The reality of that analogy is that the engines are still old... all that the analogy delivers is potentially reduced oil consumption, not improved performance. Not convinced it's true ... thicker oil is meant to be less likely to run down worn valve stems or be drawn up through the rings. At 100C all engine oil is pretty thin and one type over another is not actually going to make that much of a difference to a worn engine...