I used WD-40 spray once to clean my freewheel. It did a great job; the trouble is, some of the debris got lodged inside and prevented one or more of the little pawls from engaging in there, and I had to have the freewheel flushed under pressure at the bike shop to fix it. My LBS guy said there was no guarantee it would be fixable, but after extensive flushing it was OK. Minimal charge, but it was a nuisance.

Of course, I think spraying any solvent could have led to the same unhappy result.