It's spec'd for 5w20... but I really don't think it's going to be enough. 2005 Mazda 3s 2.3L I'm about 75 - 80% highway driving (70-75mph), 60mi round-trip to & from work five days a week... so I think that it might be a pretty nice idea to get a "thicker" 30w oil in there... but I'm not sure. I'm currently on a run of the Mobil 1 EP 5w30, and it seems to be running well. It really quieted down the engine over the Motorcraft 5w20 that comes standard... and I don't notice any MPG difference. What do you guys think... lay it out for me... GC in my Mazda? Good or bad???