I also own a 99 Legacy Outback with a multi-year history of AT slipping in low gear. I finally fixed it (at least for now!) Below is a brief history of my problem and solution (thanks to Rooster2):
* 3 years ago...transmission slips in low gear every so often...not a big problem
* 2.5 year ago...changed transmission fluid (didn't fix problem...but doesn't get much worse either)
* 6 months ago....slipping is more often and lasts longer, so I added Lucas slip and leak stop and change transmission filter (doesn't fix the problem).
* last 6 months...problem gets worse and slips all the time
* 2 weeks ago...drain a quart of AT fluid and add Trans-X. (I now have a mixture of AT fluid, Lucas and Trans-X). Slipping has entirely gone away! I'm shocked...This was my last attempt at fixing this problem before going to an AT shop for an expensive repair/replacement. I'm convinced that the Trans-X fixed it.
I will come back to this website if the problem returns and report back. But, in the meantime, I am a very happy camper!!
I've been reading this website for a while and thank everyone for taking the time to add you advice and experience.
