Ho recuperato una informazione che avevo inviato tanto tempo fa (Fri, 15 Mar 2002) ad altro Forum (parlando proprio con Carlo...il mondo è piccolo ) a proposito di GL-4 e GL-5:

Riporto quanto tratto dal sito della PENNZOIL (americana), ma puoi trovare
la stessa informazione ripetuta tonnellate di volte in giro per il Web:
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2. What happens if API GL-5 gear oil is used in an API GL-4 gear oil
application?

API GL-4 and API GL-5 products typically use the same extreme pressure (EP)
additive system, with the API GL-5 having about twice the concentration of a
API GL-4. In service, these additives become active under extreme load and
temperature when the protective oil film can be squeezed away. EP additives
work by forming wear-resistant compounds with the metal of the gear tooth
surface. As the gears mesh, these compounds shield the gear teeth from
direct metal-to-metal contact that would cause wear and damage to the gears.
If too little of the active additive is present, proper protection would be
compromised. Too much of this additive could cause excessive chemical
corrosion of the gear surface. If an API GL-5 gear oil is used in a
application where API GL-4 gear oil is called for, chemical corrosion of
"yellow metal" components may occur, such as bronze synchronizers, brass
bushings, etc. This may lead to shifting difficulties or shortened equipment
life.
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