Gear oils are designed and rated based on the type of gear system and the pressure in that gear system that the oil wil be used in, for example a hypoid setup will have to have GL5 but spur gears especailly those that are synchronized using yellow metal rings, like those in the AX trannys, require a GL3 oil.... GL4 for example has half the Extreme Pressure (EP) additives as GL5.. so using GL5 in a GL4 COULD be possible as long as the internals of the gear system will not be damaged by the additives in GL5. heres a quote from Amsoils site....
"
The differences in gear design create the need for significantly different lubrication designs. For instance, hypoid gears normally seen in automotive differentials require GL-5 concentration and performance of extreme pressure additives.
"This is because of the spiral sliding action that hypoid gears have," said Dinwiddie.
In differential applications that utilize hypoid gears, AMSOIL typically recommends one of the following: SEVERE GEAR Synthetic Extreme Pressure 75W-90 (SVG), SEVERE GEAR Synthetic Extreme Pressure75W-140 (SVO), SAE 80W-90 Synthetic Gear Lube (AGL), Long Life Synthetic gear Lube SAE 75W-90 (FGR), or Long Life Synthetic Gear Lube SAE 80W-140 (FGO).
Most manual transmissions have helical gears and do not require GL-5 performance.
"The helical gear is almost a straight cut gear, but on an angle," said Dinwiddie. "There is spiral action and very little sliding action, hence there is less need for extreme pressure additives."
GL-4 gear lubes have half the extreme pressure additives of GL-5 lubes.
In manual transmissions utilizing helical gears, AMSOIL typically recommends one of the following: Synthetic Manual Synchromesh Transmission Fluid (MTF) or Synthetic Manual Transmission and Transaxle Gear Lube (MTG)."
...heres the link to that site (I'm sure alot of people have it)... alot of good info there
http://www.upmpg.com/drivetrain/gearlube/