the R12 is the one to use.
It depends on the software whether a format works or not. I only use R13 from Illustrator, it is the only version in which object size and drawing units are exported correctly. You have to test every software. From Autocad itself I export R12. CamBam exports version 11/12 (LT R1/R2).
I always start testing with the oldest possible version. External references, blocks and similar should not be in the file. CamBam sometimes has problems with binary formats as well. Also splines cannot process some programs in DXF.
But these are basic rules if you give DXF files for further processing.
Let's wait and see what maurits writes.
ralf
@Eddy: Already gone again. Now you have to write 1111 new ones