Previous owner used 20-50 synthetic Valvoline. I'm a mobil fan all I can find local is 15-50 Mobil one synthetic. The engine is a 598cu inch with an oil cooler.
What is the difference between 15 - 50 and 20-50?
Will it hurt to mix the 2 oils I'm sure i can not get it all of the old out?

Thanks

It will not hurt anything to mix them.

It won't hurt anything to mix, the Mobil 1 will be a little thinner at startup. I would run the same oil (Mobil 1) if I were you.