Sometimes a shop with a lot of products will not be able to successfully use the Preview tool. This is because the tool will timeout if it is unable to find a single product that fits the specified conditions within 60 seconds. During this time the tool will search through approximately 5000 products.
This timeout does NOT apply to bulk tasks - only to the Preview tool. If your shop has a large amount of products, the best way to avoid a timeout when previewing is to add conditions to narrow the search.
Adding conditions will help narrow things down so that the Preview tool can display the products you are looking for. Once you need to run a task, you can simply remove these conditions (as there is no timeout for bulk tasks).