So I am originally starting this thread as a centralized location for all the questions that I have had about gear oils. I originally switched out my transmission gear oil for mobil 1 synthetic 75w90 at the recommendation of a friend. I enjoyed the new smooth feel of it but absolutely HATED the 2nd to 1st grind that came with it. I figured I might be able to live with it - after a few months I decided I couldn't take it anymore. Someone else recommended Red Line over the mobil 1 synthetic - specifically shock proof super light weight. I went to a local Red Line distributor and all they had was MTL, MT90, 75W90, 75W90NS and some others that I don't remember. My service manual called for 75w90 and I just wanted something that would work so I got the regular old 75w90 Red Line. I changed it out and the 2nd to first grind got worse. Why? Well, I decided to do some research.
Why does it grind after putting the new oil in?
great info on this *here*
It's interesting to note that the type that Cobb Tuning has the "highest recommendation" for is Red Line MT-90, which just so happens to be GL-4 rated. More on this below.
According to raceshopper.com:
Red Line MTL is classified as a 70W80 gear lubricant satisfying the API Service requirements of GL-3 and GL-4. MTL also satisfies motor oil viscosities SAE 5W/10W/30, and the viscosity requirements for ATFs. MT-90 is a 75W90 GL-4 and also satisfies motor oil viscosities SAE 10W/15W/40. The additive chemistry used is non-corrosive, so even though they will provide GL-4 performance, they will not corrode synchros or bushings. Because of its low corrosivity, Red Line MTL and MT-90 could also be used in transmissions requiring GL-1 or GL-3 lubricants. Seal compatibility is designed to be similar to petroleum-based lubricants.
What is GL and MT-1 rating?
The gear oil's GL rating as set forth by API measures how well it will perform under extreme pressure. Gear oil is manufactured with anti-wear additives to stop metal surfaces from welding together. The GL rating ranges from 1 to 6. Gear oils rated 1 to 4 are considered anti-wear products, whereas GL 5 and 6 products are for use under extreme pressure.
Denotes lubricants intended for axles with spiral bevel gears operating under moderate to severe conditions of speed and load or axles with hypoid gears operating under moderate speeds and loads. These oils may be used in selected manual transmissions and transaxle applications where API MT-1 lubricants are unsuitable.
Denotes lubricants intended for gears, particularly hypoid gears, in axles operating under various combinations of high-speed shock loads and low-speed, high-torque conditions. Lubricants qualified under MIL-L-2105D satisfy the requirements of the API GL-5 specification, although the API designation does not require military approval.
Denotes lubricants intended for nonsynchronized manual transmissions used in buses and heavy-duty trucks. Lubricants meeting the requirements of API MT-1 provide protection against the combination of thermal degradation, component wear and oil seal deterioration. API MT-1 does not address the performance requirements of synchronized transmissions and transaxles in passenger car and heavy-duty applications.
GL-5 has the following oil performance tests with stringent requirements:
Gear test using differential assembly measuring the resistance to corrosion in the presence of moisture.
Gear test using complete axle assembly measuring the resistance to gear distress under low-speed, high-torque conditions.
Gear test using complete axle assembly measuring the resistance to gear distress (scoring) under high-speed, shock-load conditions.
Bench test using spur gears measuring oxidation stability.
Bench test measuring foaming tendencies.
I also found this information from Red Line:
Most manufacturers of manual transmissions and transaxles recommend an 80W or 90W GL-4 lubricant. GL-5 gears oils which are required in hypoid differentials are not used in most synchromesh transmissions because the chemicals used to provide the extreme pressure protection can be corrosive to synchronizers, which are commonly made of brass or bronze. Typically, the use of a GL-5 lubricant in a synchromesh transmission will shorten the synchronizer life by one half. The extreme pressure requirements of spur gears and helical gears found in transmissions are not nearly as great as found in rear-wheel drive differentials. A GL-4 lubricant provides adequate protection for most manual transmissions, unless a unique design consideration requires the extra protection of a GL-5.
This leads me to believe that Subaru decided that this "unique design" was given consideration and requires the extra protection of a GL-5. So why does Cobb Tuning recommend a GL-4? That answer I do not have. What I did surmise from the above quotes is that the higher the pressure protection (GL rating) the more corrosive it is to brass or bronze synchros. You'll probably have to re-read the above quotes (in italics). It's okay... I'll wait...
What is API?
API is American Petroleum Institue. They set forth what they call service ratings (GL, MT, etc...) described above.
What are 02+ wrx synchros made of?
Bronze according to various threads on nasioc. No cold hard evidence yet but most say
they are bronze.
What is SAE rating? (ie 10w30, 75w90, etc...)
I would say the good majority of people know what this means but in case you do not, go *here*
So which gear oil should I use?
I know you were probably hoping for the tell all answer from all of this but my hope is that this is educational enough for you to make an educated decision based on the information provided. If it means anything, I am probably going to go with Red Line 75W90NS unless I find something that looks like a better compromise (gears/synchros) with GL-5 rating.
I hope this helps everyone out at least some.
More information on oil properties / constituents:
Gear oil used by Subaru dealerships (OEM)
Valvoline (non-synthetic) GL-5 75W90
What I ended up doing
I put 75W90NS (Red Line) in and shifting got a lot better. It doesn't grind anymore into first but I cannot shift into first at very high speeds still. It simply made the lockout from first work a little better and prevent me from grinding into first. I still rev-match to get in at higher speeds.