Sort products by newest to oldest

Hi, I'm currently running zen cart version 1.3.7.1 and having an issue with the product sort order.

Basically my products are automatically listed alphabetically depending on each individual item's 'sort order' (when entering a new product). For example, everything with sort order 0 will be listed first alphabetically, then those with sort order 1, then sort order 2, etc.

This is fine for the category listing but not for products. I want the products to be listed with the newest items showing on the first page and older items placed at the end.

I've gone to "admin>configuration>new listing>display product display" and have set this to "6" (date added desc) but this has had no effect. Changing this to any value still doesn't change my product listing order.

Could it be that i have to change the php template file(s)? If so, which file would need to be changed (and changed to what?)

Please can someone help! ^^

Thanks in advance!

I had always thought that the idea of 'sort order' was to give each product a unique value, so you could start with, say, 5000 and give each newer product a lower number, which should place the newer items at the top.

that's what im doing at the moment. I give everything a sort number, say 100, then work my way down... so 100 = very old items, and 10=newest items. But the problem is that eventually everything in sort order 10 will become 'older items' and the problem starts up again. Is there no way to change the priority of the sort order? i.e. having the higher sort order appear first?
1=lowest priority, and 100=higher priority (101=even higher, etc)

Thanks again.