Wow! It's pretty interesting to read all of the opinion, fact, rumor, and myth about our products and their success in the Subaru transaxle. Our customer service man Dave Granquist is right in his reply off of our site--75W90NS is the GL5 oil of the correct weight and the 75W90 is for the rear. That's why it's part of our PowerPack for these cars. Plus, your Subaru dealer can't give you any grief over using fluids that aren't designed for the task.
Someone made a smart point? You have to sit back and think, "Maybe my transmission is the problem?!" These transaxles are quite frail in comparison to the pounding and quick shifts you put on them. The oil is often the last thing people change, usually after they are trying to fix a grinding problem... We think some of the mixed results you guys are seeing with GL4 and GL5 fluids of different weights is based on the particular conidition of your transaxle. Our experience with WRX shows that a mechanically sound transaxle can switch to our fluid and have better shifts, better cold operation, put more power to the ground, and get longer life.
The Cobb guys have a lot of tranny experience, but they are quite wrong in their recommendation on their site--our 75W90NS is NOT the best fluid for a rear differential. Our regular 75W90 has some friction modifiers, but it doesn't get in the way of the non-clutch LSD's operation and the friction modifiers actually help the gears a bit.
The biggest difference between the GL4 and GL5 oils is the amount of EP or extreme pressure additives in the blend. It's not a street spec and a racing spec. The GL5 oils usually used for differentials with a hypoid gear, but like in the case of Subaru, they are often a tranny or transaxle spec.
ShockProof is a unique fluid that works a bit like a liquid grease with a microscopic medium--it works well in some street applications and not in others. If there is a pump, screen, or filter in the mix, it usually isn't the right fluid. Street tuners with a lot of savvy use it, but it's on a trial basis. Admittedly, even those of us with a lot of experience have a tough time tracking which fluid is working better for a LSD at any one time, so it's tough to know when it is hindering the process. Superlight is a 70wt, Lightweight is an 80wt and Heavy is a 90wt and these fluids can be mixed to come up with custom viscocities. We haven't done the same testing with ShockProof on the Subaru transaxle like we've done with the 75W90NS, so it's cool to hear that it is working for some of you.
Hope this helps. You guys have a cool site...
Red Line Oil