Will inhaling a small amount of post nasal drip into lungs cause aspiration pneumonia?

In medicine we never say never, however, the cited issue is unlikely. There are natural automatic protective mechanisms that protect the airway. Unless the person were deeply drugged to the point these automated systems failed, it would not happen. That is why people undergoing deep anesthesia have a tube in their windpipe to protect it.

Agree with Dr. Ferguson. You are unlikely to aspirate post-nasal drip into your lungs.