As has been discussed previously in this forum, there is some question as to what is an acceptable transmission oil to use in Xterra manual transmissions.

The Nissan spec calls for a GL4 gear oil, presumably to avoid additional sulfur-based additives in GL5 gear oil that may damage soft metals in the transmission.

However, it's pretty hard to find a 75W90 GL4 gear oil--which is what Nissan recommends. It is easy to find a 80W90 GL4 oil, but for people that live in cold climates like I do, 80W90 is just not an option.

I became concerned because I have been using Valvoline SynPower (full synthetic) gear oil in my transsmission. It's a GL5 oil.

I called Valvoline (800-832-6825) and spoke to a tech person and was told that SynPower uses a "non-active" form of the sulfur additive that will not damage any soft metals in a transmission and that SynPower was safe to use in any application that requires GL4.

Finally, while SynPower is probably safe, it's also a lot more "slippery" than non-synthetic gear oils, meaning that you may have to shift more slowly (expecially between first and second gears) to allow the synchronizers to "catch up" because the gear oil is so slippery. That would apply to any full synthetic gear oil.

I live in Gunnison, Colorado, where it's quite cold for much of the year. When it is cold, my transmission (with Valvoline SynPower) is totally perfect. However, in the summer months, I find I do have to shift a little slower than I would in the winter, especially from first into second.