You can still bring in cars from SP To MP but if there to hot to handle then you can't store them as the tank method has been patched. Hope this helps

working as of 1.10 did today

Thank you, this helps! Worrief they mighta patched it hehe :p, theroosevelt will be to hot to store, right?

Yes it's to hot to store sorry

Can anyone help me out? Whenever I'm about to bring the car online( the step where you sit in LS customs) I proceed to a private session and it says 'failed to sync files with GTA Online.Please return to Grand Theft Auto 5 and try again."

Well uh, I use a different method so.. You could try that method?
Step 1 - Stand outside your apartment on GTA online and make sure your spawn location is on last location
Step 2 - Load up single player with any character, as this will save your online progress
Step 3 - Start up an Invite Only Session and make sure that you spawned in the correct area
Step 4 - Now go to the xbox games store and on the second zoom out of the camera press the guide button, then sign out and back in
Step 5 - You will load up the prologue mission, when this happens press start, game, load game, and load the save with the car you want on it
Step 6 - Get in the car you want and park it close to your spawn point on GTA Online, in this case, outside your garage.
Step 7 - Load up an Invite Only Session, when you load into the game you should spawn outside your garage/apartment and the car should be where you left it

Hope this helps!