Well, Subaru high performance gear oil (this is not extra-s, extra-s has been superseded.) is not synthetic based, while Redlines 75w90 fluid is. I'm not a fan of mixing fluid; i'd just go back to the dealer and pick up some high performance gear oil, which is probably what they filled your gearbox with. If you have the paperwork or service records, you should be able to call and see for sure what fluid they used.
Certain dealers will just use some random GL5 bulk gear oil, certain dealers use Subaru OEM stuff, it all depends, there's really no standard or quality control in place for that.
TL;DR: I wouldn't use Redline.
Also, for future reference; I know this is kind of off-topic. I haven't heard much good using straight redline, these 5MT boxes house both syncros and the diff in the same fluid; so they're extremely picky on the type of fluid that works well with both syncros and differentials. I use straight Motul Gear 300 75w90 on my 5MT, that stuff shifts like butter in the cold. When I did my 90k fluid change, I filled the transmission with Subaru high performance gear oil, which, in my case, was notchy, caused 5th gear to grind, and felt like mud in the cold. I quickly (in 3k miles) drained that crap out, and have been running Motul Gear 300 for nearly 25k miles without issue.
Edit: Jack your car up so that the fluid rests on the opposite side of the axle that you're pulling, you should lose a very minimal amount of gear oil when you pull your old axle seal out.
Last edited by Akasa; 10-23-2015 at 09:53 AM..