is thinner than "standard" gear oil. This lets the gears spin up faster. If the syncros are warn out, they are not going to "get better". A tranny rebuils on our boxes is NOT an easy job. The bearings are hard to remove, and reassembly is tricky, needing alot of "feel" to get the freeplay correct. I did mine, with assistance from "experts". I could NOT have done it alone. Best left to the experts.