03 Mustang v6 .....Rpm dropping/ New oil Change

[03 Mustang Guy]

I recently purchased a 03 mustang v6 and i did a oil change but i think i might have put to thin of a oil change 5w-30... The car has 150,00 miles on it..i changed the oil to Valvoline High mileage a synthetic blend and and the very next day i began to drop Rpm and the engine to have a rough start with rpm dropping and rising and after a while it settles and the car idles good but also it feels like the engine doesnt have to much power as it did before...im planning to take the car back to the shop for another oil change to conventional oil of 10w -30... Whats a good conventional oil to use and you yall think that might be my problem..someone please lmk asap