If the previous owner used 20-50 id be wondering why.
Has he tried to slow down a leaky engine or an engine that is burning oil with thicker grade? Personally, if he told me that id be running for the hills.
Alloytecs need good synthetic oil and regular changes. Chances are, if the previous owner has been chucking 20-50 Mangtec oil in it, its going to be a ####ing sludgefarm in there.
Owners book recommends a 5-30 IIRC. Id be looking at a good quality synthetic, 5W30 like HPR5 or even a HPR10 10W50 if its a little noisy. Do a couple of really good flushes/quick oil changes and dont touch Valvoline with a 10 foot pole.