Originally Posted By: DeepFriar
Key words in that thread? - "Metal deactivators"
Requires a degree of trust then if you use multi-GL instead of GL-4 where GL-4 is specced. With RP, for instance, having gone out of their way as I said earlier, to state that theirs is safe for GL-4, I would tend to "trust" them. I might be less inclined to trust many others.
I don't know why one would trust RP over any other Manf. who makes the same claims.
I think we have to look at the targeted application.
What fluid type and protection level do most manual light truck and car transmissions and transaxles require?
It is a a
dedicated MTF with a
GL-4 level of protection that can be accomplished by using a medium level of AW additives consisting of ZDDP and other boosters with a specialized friction modifier.
What level of protection do the majority of modern differentials require? It is a GL-5 level of protection.
This is accomplished by using an EP additive package consisting of S-P and boronated additives.
It is a fact that today's EP chemistries are 'buffered" using metal inhibitors and anti-corrosive additives. What this means is that yellow metals such as copper (brass/bronze) alloys don't get attacked.
When was the last time you saw any modern hypoid differential that contained anything but steel components?
But the GL-5 lube could also be used in worm drive automotive and industrial gearboxes containing copper (brass/bronze) alloys because of the chemistry described above, even if the gearbox's protection requirement was GL-1, GL-2, GL-3, or GL-4.
So the answer "yes," it can be and is formulated for that range of protection levels.
Quote:
The GL-X rating defines the protection level for gearing, whether it be transmissions, TC's, or differentials.
A GL-5 rating is for highly-loaded gears (high tooth contact forces), such as for hypoid differentials, and contains high levels of primary and secondary Extreme Pressure (EP) additives such as Sulfur-Phosphorus chemistry. The viscosity of said GL-5 gear lubes is usually higher than for manual transmission lubes. For example, a 75W90 GL-5 gear lube has a viscosity of 18.5 cSt@100C, whereas a GL-4 protection rated 75W90 Manual Transmission lubricant would have a viscosity of 14.0 cSt@100C. This difference in viscosity can have a major impact for Manual Transmission shifting performance in terms of cold weather shifter fork movement.
A GL-4 rating is for lower loaded gearing such as for manual transmissions (MT) and Transfer cases (TCs) in light trucks and cars. It contains mostly an Anti-Wear additive (AW) in the form of ZDDP or an equivalent anti-wear package. It does not contain Extreme Pressure (EP) additives. Furthermore, the chemistry for MT's contain specialized friction modifiers for the synchronizer assemblies.
The GL-4 Performance Improvement package (additive package) has been tested by the additive suppliers to perform to GL-4 protection levels using modern testing machines.
For manual transmissions that share the same sump with hypoid differentials, such as the Subaru drivelines, a special GL-5 lubricant is used which has the synchromesh friction modifier. But this is an exception.
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