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OK I'm back with the continuation.
The GL5 additive package has been blamed for damage to copper-bearing metal parts in transmissions. These parts are synchronizer rings which are made of brass. The returns are very mixed here. If EP = GL5 as I believe, many manufacturers recommend EP/GL5 lubricants in their transmissions, including Saab (as cited above) and Toyota. It would appear that either the additives in the GL5 package have been changed to no longer cause damage to
copper-bearing components, or said components have been changed to resist the damage.
The additives in question are phosphorus and sulphur compounds.
I believe the current generation of additives do not damage the rings. I have found one website (search "GL5 gear oil definition" supporting that position.
The effect of damaging additives on damageable components will depend significantly on operating temperatures, as chemical reactions generally are accelerated at higher temperatures.
Synthetic oils have the virtue of reducing operating temperatures.
Bottom Line: Redline MTL which is widely recommend, and is what I put in my V4 transmission, is a GL4 lubricant and not a GL5 lubricant as apparently (if "EP" = GL5") specified by Saab. Redline 75W90 is a GL5 lubricant and I don't like the 90 number for vintage Saabs.
Lubricants of today are head and shoulders above what was available 20-30 years ago.
Personally I think either will work fine. I have never encountered a real-life example of synchro damage from EP/GL5 additives. I presume this has happened but is likely related to other circumstances such as elevated temperatures, racing use, extended use beyond appropriate change intervals, stuff like that.
posted by 208.53.81...
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