RedLine 75w90ns is *not* Shockproof!
Here's a summary of the Redline gear oils applicable for our Subarus:
Redline 75w90ns:
"This GL-5-type geal oil doesn't contain the friction modifiers for limited-slip hypoid differentials. This makes the transmission synchronizers come to equal speeds more quickly, allowing faster shifting and much easier low-temperature shifting. Can also be used in racing limited-slip differentials where weak spring design causes too much wheel spin."
This is good for the tranny if you're concerned with staying with a GL-5 oil. Frankly, I think it's too slick for the synchros, and will cause the tranny to shift poorly unless you're willing to double-clutch and rev-match more often than not.
Redline 75w90:
"Our most popular gear oil, this is the preferred product in nearly all car and light truck differentials, both conventional and limited-slip. 75W90 is preferred for most racing applications. It contains friction modifier which is recommended for limited-slip units. 75W90 Gear Oil can be used in many transmissions and transaxles; however, other Red Line lubricants have better frictional properties for rapid synchronization. Exceeds API GL-5."
I use this in my rear-diff. But it's not for use in the gearbox, due to the poor synchro performance that would result.
Redline Lightweight Shockproof:
"A unique gear oil designed to lubricate racing transmissions and transaxles which see serious loads (not recommended for most syncro-type transmissions). It has excellent low-temperature flow which allows easier shifting when cold. May be used to obtain maximum power transfer in racing differentials which do not see high temperatures. Similar to a 75W140 gear oil, but with the lower internal friction of an SAE 30 motor oil.
Used in two-stroke and four-stroke motocross transmissions (often mixed in equal parts with SuperLight ShockProof™ Gear Oil).
Used in most road-racing differentials with moderate power and dogring racing transaxles."
This is what I'd recommend. I'm actually using a 50-50 mix of Lightweight and 75w90ns, but I've run the straight shockproof before and liked it better (better synchro grab).