"no ceramic" is painting with too broad a brush
NP0 is a low k ceramic dielectric that gives near perfect distortion performance excepting for some leaded types possibly having bad end metallization/lead attachment issues
Mica is a very variable natural material that can be as poor as better Mylar
and Mylar/Polyester is the worst common plastic film dielectric - Polypropylene is better, Polystyrene measures best for low audio frequency distortion
Cyril Bateman's "Capacitor Sound" series did serious measurements, Bruce Hofer of Audio Precision gives simailar recommendations in his seminars/presentations
the
http://www.diyaudio.com/forums/equipment-tools/205304-low-distortion-audio-range-oscillator.html has many measurements, references, commentary if you really want to get into a analog oscillator build
I would just recommend a decent soundcard like the ESI Juli@
from the diyAudio thread, showing 24 bit mode may be necessary to even see the distortion: