My new Quality Manager has decided that all Customer returns should be regarded as Customer complaints, I'm not sure that I agree with this though.
Why are you unsure about whether or not to agree with this definition?
Depending on the nature of your business, a return IS a complaint, however in some cases the return may be due to the Customer acquiring the wrong part and returning it because of an error on their part. In other cases, the return may be due to a defect in the product.
What comes after the return is categorizing the nature of the complaint and determining whether or not the criteria has been met to warrant further investigation.
For example, a $5 part returned due to a defect may have less attention than a $5,000 part. Although, if that $5 part is a key safety feature (e.g., consider the $10-$20 plate that Ford engineers needed to make the Pinto a safe car to drive in the 1970's), it may garner more attention than a more expensive, yet less critical, part.
Treating every return the same would, in my opinion, be the incorrect approach to take, but the initial classification of 'complaint' isn't worth getting to upset over. It's all in the process for categorizing and handling the complaints.