You can only play the new Gran Turismo 5 DLC using the PSN ID you bought the content with - it's locked to a single account.
The Gran Turismo 5 website confirms this:
Gran Turismo 5 creator Kazunori Yamauchi was inundated by fans asking why on Twitter.
"Resolving the problem with using the DLC on multiple accounts is not simple," he said.
"It would be best if we could attribute the entitlement to use the DLC items to multiple accounts in the same PS3.
"But there is nothing like that in the PSN setup, and we can only assign the entitlements per user ID.
"There just isn't a good solution for this I can think of."
The Gran Turismo 5 DLC comes either in one giant, £9.49 pack, or piecemeal.