Maybe look for a gear oil with a low pour point, 40C viscosity, and low brooks (most gear oils have these in their datasheets); that may point you in the right direction especially if multiple 75w-90 oils are offered by the same company. Not sure if that actually works since a lot of other properties affect synchronizer performance but maybe worth a try.500 miles to go -- going to put Amsoil in it this time.
Forward drive gears are actually always meshed at the same time in most manual transmissions. When you select a forward gear you are usually engaging a clutch for that gear (called a dog) that locks the selected gear to the shaft while all the other gears spin freely on the shaft. For the reveres gear you do in fact actually mesh a gear in. The teeth on a gear must be made much thicker than the teeth of a dog because the teeth for a dog are engaged simultaniously while on a gear only a few are so the loading per tooth is much higher. That is why it is harder to mesh the reverese gear than dogs gears. It is common with many manual transmissions.reverse dead cold is sometimes hard when the car is still off, putting it into other gears rectifies this, which leads me to believe it has to do more with one 'tooth' (for lack of better word) is narrower/wider than all the others by enough to make it rough and vice-versa on the other side.
If there is actually an issue with the first gear maybe shouldnt cancel the warranty since the risk is high that the problem will grow to the point that they wont be able to deny the issue. Instead get a quote for a repair from a shop for proof of problem and file with the BBB and your states attorney general; pretty effective.TM actually has a unique setup, not in that he has a problem, just that the dealer gives half of a crap about it . . . I've given up, I'm sending Mazda a letter asking for my money back on the extended warranty based on the fact that they can't seem to fix even little things.