Itunes uses the AAC format and a "Digital Rights Management" (DRM) called Fairplay. This is essentially there to stop you making copies of/giving away etc etc tunes you have bought. As wikipedia puts it:
<italic>FairPlay will allow a protected track to be used in the following ways:
* The protected track may be copied to any number of iPod portable music players.
* The protected track may be played on up to five (originally three) authorized computers simultaneously.
* The protected track may be copied to a standard Audio CD any number of times.
o The resulting CD has no DRM and may be ripped, encoded and played back like any other CD. However, CDs created by users do not attain first sale rights and cannot be legally leased, lent, sold or distributed to others by the creator.
o The CD audio still bears the artifacts of compression, so converting it back into a lossy format such as MP3 may aggravate the sound artifacts of encoding.
* A particular playlist within iTunes containing a protected track can be copied to a CD only up to seven times (originally ten times) before the playlist must be changed.</italic>
Therefore, I would just burn it as a standard audio cd using Itunes, and play that in the car. If you really have to make an MP3 version, you can always rip that cd back into mp3, but you will lose quality.