At higher RPM the rotating parts tend to actually stretch a bit. If you buy a used car driven at slow speeds all of its life you will find an abnormally large ridge at the top of the cylinders because the piston rings never get the high. That's why the old lady car often starts smoking or blows not long after you buy it. You ruin the rings on those ridges. This problem has become much larger over the years with the majority of cars having automatic transmissions and nowadays the shift points are quite low as well.
Start using synthetic oil. It costs more but lasts much longer while doing a better job