Our '09 Q5 3.2 had a drain plug leak that took me a while to figure out. I'm sure it isn't common, but I've read of similar issues and wanted to pass this on.

On my first oil change, (used car, 100K miles) when I already was aware of the leak, I found the drain plug's aluminum crush washer had been so over-tightened that it was oddly distorted. So I figured that a new copper washer would solve that, but it still slowly dripped, although the pan looked like new and I couldn't see any sign of a crack. I next went with a new factory aluminum crush washer, which still dripped after torquing to 22 Ft/lbs. When that didn't work either, I planned to install another crush washer, plus a new drain plug, and use a thread sealant this time. With all the oil drained, and cleaning the threads in the pan, I spotted a small break in the threaded fitting, right at the first thread, which would be opposite the necked-down part of the drain plug. This might have been a manufacturing defect, or resulted from the over-tightening. So instead of thread sealant, I applied an automotive Silicone gasket sealant on the drain plug threads and both sides of the washer. After again torquing to spec, the leak looks to be gone.