I don’t usually regurgitate what others have said, but in this case, it seems needed. SingleFeed has supported the Product_Type field since well before it was required. And for some reason this is the field we get the most questions about. I like to describe the field as exactly what the product is. Google goes a little further, and I thought this might help merchants. From the Google Base Blog:
“The product_type attribute is used to categorize items so that we can match them to search results. Entering product types that are too general or that are incorrectly formatted causes our system to categorize your item incorrectly, and we won’t be able to connect your product with the right searchers. So when you enter the product_type attribute, you should think specific and relevant. For example, if you’re selling keyboards for computers, you should include “keyboards” as the product_type and not “computers.”

If your product doesn’t fit with the exact values on the example product_type page, you can include some variations; the most important thing is that the product_type be relevant and that it distinguish between an actual product and the accessory for the product. For example, for the above keyboard example, the product type “usb keyboard” is also acceptable. If you’re selling Ipod skins, the values “accessories” or “mp3 accessories” are fine.

One strategy for figuring out your product_type is to take the last breadcrumb from your categorization tree. So if your product page has the categorization Camping-Hiking > Sleeping Bags, it’s a pretty safe bet (although not a sure thing) that the product_type is sleeping bag.

If all else fails, try using this list provided by Google Base.