It is, of course, much more viscous. If there is no problem with it getting to all vital areas it will most likely add a lot of life to the tranny, but you may sacrifice some shift smoothness and gas milage. The problems with using gear lube where ATF is recommended are generally due to the inability of a pump (if so equipped) to move the heavier gear lube and for it to travel where needed. In a transmission this may not be a problem like it can be in transfer cases.