Ok, never mind.
The user manual says: MP3, WMA, WAV and ACC (probably a typo, should be AAC).
It probably plays back WMA10, which is the encoder I used to encode my files (and hope it plays back VBR WMA as well).
WMA VBR Q0 44.1kHz equals about 35-67 kbits of audio, compares to MP3s of 128 to 160, meaning the low compression is good for audio books, while the high one gives near CD audio quality.
Anyway, I'll be testing out some formats, as well as large file size compatibility (my Cruze doesn't play back large files well).