I like 80w140 synthetic in rear ends. I use 50 wt synthetic gear oil in my transmissions. However, many consider it too slick for synchronizers to work properly. The transmissions were built for 90 w straight mineral gear oil and they ran a lot of miles with that. If you are willing to spend the money and really want a synthetic in your transmission, this stuff is good. http://www.redlineoil.com/product.aspx?pid=46&pcid=7 You will probably hear as many different opinions as people you ask.