As posted once before, here is a copy of an e-mail I got from Castrol concerning the GL-4/GL-5 conundrum.
API GL-5 gear oil designates service of hypoid and other types of gears commonly found in rear axles. A GL-4 oil may be employed in certain manual transmissions.
These differences are mainly distinguished by the level of active additives employed in the product. Different applications require different additive levels and different component materials can be affected by the levels of additives present in the lubricant.
Having said that GL-4 and GL-5 gear oils are not compatible and manufacturer's recommendations should be adhered to. They cannot meet the same specifications and the product performance and tests are different.
Typically, a GL-5 gear oil will have about 2 times the active additive level of a GL-4 product. This additional additive can cause problems with yellow metals like brass or bronze.
Castrol Syntorq LT is a premium high performance, synthetic gear lubricant for synchronized manual transmissions. This product is manufactured specifically for GM and Chrysler and is currently available through their dealerships.
Castrol Consumer Relations.