Here's something I picked up from the mazda.ca site...when creating and owner's homepage u gain access to FAQs.
Can I use synthetic oil in the engine of my Mazda vehicle?
Mazda has not done any testing related to the use of synthetic oils and therefore does not recommend the use of these products. However, using this type of product does not, in itself, affect the warranty on your Mazda. As Mazda vehicles are not designed with the use of synthetic oil in mind, any negative impact on the vehicle as a result of using synthetic oil will not be covered by warranty.
So I guess u can use it but like insurance compagnies, they'll try and screw u the first chance they get to save a buck on their end.