Need help converting from MPEG 4 to MPG 3 [View all]

I bought a new Subaru BRZ and it has a port for plugging in a flash drive to play music. So I bought a bunch of new stuff off iTunes and loaded it onto a flash drive, along with some of my older tunes. Then I learned that, unfortunately, the audio system won't play MPEG 4, it only plays up to MPG 3 from a flash drive. I tried converting the 4 music to MPG 3 in iTunes and then loading the converted MPG 3 onto the flash drive. I was able to do a conversion and get both the 4 and 3 versions of the songs, but I'm still having trouble getting it to play so I'm not sure I converted the music correctly. When I load the music onto the flash drive, I'm having to load it off the hard drive's music folder. I can't load it off iTunes. I'm using OS 10.6.8 (Snow Leopard, I think) and iTunes 10.6.3 (25). When I put the flash drive in the car's player, the drive shows 2 versions of each song, on preceded by a number, then a ._, then the name of the track. The audio system won't play any of the tracks in that version, but will play the tracks without the #._ in front of the song name. When I open the drive on my computer, only 1 version of each song appears.
Can anyone help me figure out what I am doing wrong. How should I be converting the songs? Should I be doing something else to the 3's to get just the 3 version on the flash drive? How can I delete the #._ versions from the drive when I don't even see them on the computer? I've tried wiping the drive and reloading the 3's but it still comes up with 2 versions on the car's audio system. The drive comes up fine in my neighbor's Acura MDX audio system so I don't really know what the problem is. I really am kinda' tech challenged, not really up on all this stuff so really need help. Anyone have any idea's?
Thanks in advance for any help you can give.
ps: please be kind, I'm really trying to figure this out.