Yogurt only helps if has live cultures in it, it doesn't matter if it's normal or Greek, as long as it has those cultures. :]
I've personally never dealt with sour crop, but here's some good info...
It's normal, when it warms up it melts. Probably says something like that in the back of the jar.
it won't do anything for sour crop, though. But it's great for impacted crop