If the manufacturer specifically states it is compatible with a synchronized manual gearbox (and you believe they'll stand behind that if your synchros are destroyed) then have at it. I have no reason to believe yours isn't perfectly ok -- I'm sure it is -- but carrying both ratings on the label by itself IMHO is not enough because in a hypoid-gear differential said dual rating has no chemistry or film-strength concerns.
Your last sentence is confusing and I am not sure what it says.
Again, We're discussing here a
properly formulated and application specific MTF that has a GL-4/GL-5 rating which would be safe for copper-alloy synchronizer assemblies. An MTF with a GL4-/GL-5 rating ALSO has an additional component called a "Friction Modifier" for the synchronizer assembly to insure proper meshing and spin-up forces.
(Copper-alloy synchronizers here imply a
Bronze or Brass metal machined alloy).
A hypoid gear differential lube, once again, has to have a GL-5 rating with the proper EP additive because of it high
loading. Loading here means there are high contact forces in the gear teeth and high reaction forces against the bearings and shims.
The EP additive prevents galling of the tooth contacts.
Here is what I posted on Oct. 13, 2017:
"It is NOT what is MISSING but rather the DIFFERENCES between gear fluids of different ratings: Dedicated or Application Specific MTF’s for Manual Transmission or Transaxle use an additive package containing Anti-Wear (AW) additives, rust and corrosion inhibitors, emulsifiers, and specialized Friction Modifiers, and are generally protection rated at the SAE GL-4 level. Now GL-4 does not necessarily refer to MTF’s, as there are some gear lubes in the Market place that are GL-4 rated, but are not MTF’s. GL-4 is an SAE wear protection rating. There are a few MT fluids GL-5 rated for specific vehicles that have common MT and Differential sumps. One such manufacture, Subaru, has transmissions which shares a common sump and require a MTF which is GL-5 rated; but the Subaru-type fluids also contain special friction modification chemistry for the synchro assembly. Manual Transmission fluids use a different Friction Modifier for synchro engagement, a modifier that is NOT the same Friction Modifier chemistry as used in differential lubes, engine oils, or hydraulic fluids. Here, Friction Modifier or Friction Modification does NOT refer to friction reduction. Here, Friction Modifier refers to a chemical compound that gives rise to a situation such that the coefficient of friction (COF) varies Dynamically with respect to the relative speed of parts that mesh and de-mesh during engagement and disengagement of rotating parts. In summary: 1. GL-5 and MT1 rated gear lubes have a higher viscosity than MTF's of the same SAE Gear Oil viscosity range, 2. GL-5 and MT1 rated gear lubes have a different additive package than do MTF's; MTF's contain Anti-Wear additives, GL-5 and MT1 rated gear lubes contain Extreme Pressure (EP) chemistry. 3. GL-5 and MT1 rated gear lubes have a different additive package than do MTF's; MTF's contain specialized Friction Modification chemistry; GL-5 and MT1 rated gear lubes containing LS additives have special Friction Modification chemistry that is different than what is found in MTF's."
I remember something about GL 5 should not be used in an application that has brass or copper fittings or parts because the higher level of EP additives in GL5 (vs GL4) can cause corrosion to the brass and/or copper parts in the gear box. The brass is most likely to be used for the synchro mesh...
bobistheoilguy.com
See also:
'
Gear Tribology and Lubrication - Part I General In this tech brief we will examine gear types and the lubrication requirements for manual transmissions, manual transaxles, and differentials. Gears and gear boxes are the most often overlooked component when considering oil changes and periodic...
bobistheoilguy.com
and
Gear Tribology and Lubrication – Part II Rheology and Chemistry of Gear Oils By MolaKule Gear oils lubricate the gearing teeth and the support bearings of transmissions and differentials as they transmit power, and react to forces such as shock (impulse loading), braking, and acceleration...
bobistheoilguy.com