Chris, I hope you and Mary (Or is it Mare?) stick around. We really are mostly harmless. Many of us are here because we can't stand the noise level on Nasioc, I-Club, etc. That is why we pick on Cody, Juice and others when they get too random or homophobic.
So, if she could dial down the chaos a bit, it would go a long way to keep Austin from locking her threads.
The WRX MT5 is a fragile beast if remotely abused. I put 30K of probably 25-30% track and autocross miles with no problems, but I know how to shift and only used the factory fluid and LW shockproof.
The factory fluid is some magically stuff that I believe is only available in 50 gallon drums and even most dealers don't use or carry it. Since Subaru would have to give away their fluid if they specified it, they specify generic GL-5 gear oil instead because of the diff.
The diff is not the weak point, so the search was on for something that would save the trans from braking and smooth out shifts while not sacrificing the diff. You can go read the countless threads on Nasioc on it, but the choices come down to some cocktail, LW shockproof or maybe the Motul, all with basically zero science and lots of anecdotal evidence averaging out to a crap shoot.
Try downshifting the 5MT to 1st at more than 5-10MPH or getting it in gear in the cold. If you like it, leave it. If you have to add or replace fluid, do the research and make a choice.
Personally I think the cocktails are silly as they have no science at all behind them and god knows what the additives and bases in each actually do chemically when combined. The LW shockproof or GL-5 Redline products would be my first choice because I have had great success with their products over 12,000 track miles in 4 cars and they have a very low BS/snake oil quotient in their sales and tech info.
I like Motul fluids for brakes, but have no personal experience with their oils, but would be willing to try if I didn't like a RL product.
My last choice would be anything off the shelf at Kragen or easily obtained from a mass retailer locally except Summit which carries much of the good stuff. They are one of the few bright spots you will find about Reno from a performance parts perspective vs. LV. 9:00-9:00 7 days a week, you can get a lot of good stuff.
Let us know what you end up with and how it works.