What potion did Harry Potter take to get into Horace memories?

Harry Potter didn't actually take a potion to get into Horace Slughorn's memories. He used a Pensieve, a magical basin that allows wizards to see and experience memories.

He used the Pensieve to relive Slughorn's memory of a conversation he had with Tom Riddle regarding the nature of horcruxes. This memory played a crucial role in Harry's quest to find and destroy Voldemort's horcruxes.