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.
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.
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.
good luck
