Switch back to 0w20, 0w30, or 5w30. These grades are available in fully synthetic, 5w30 you can find both full and semi synthetic.
2 oil changes wouldn't have caused any issue, don't go for engine flush. Just get the appropriate grade in the system, it'll dilute and remove any residue gradually itself.
Brand wise, as long as you get original oil any reputed brand is fine.