The Mazdaspeed 3 six-speed transaxle features what Mazda calls a "cone-type" limited-slip differential. The owners manual specs a GL-4 75W-90 gear oil, yet the factory service manual says GL-5 is also acceptable. Hmmm... In any event, my dealer replaced the factory fill with Synlube gear oil at 30,000 miles and the result is that the shift action requires much more effort when cold and, even worse, the LSD groans loudly in tight turns. As a result, I want to replace the Synlube with something that will quiet down the LSD and still provide excellent lubrication properties- especially since the car sees the track frequently. Several people have suggested that I use Ford XT-M5-QS Synthetic Manual Transmission Fluid. Even though it's a bit expensive, I'm leaning toward that option. However, I would like to know if anyone else has some other suggestions. Thanks!