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GL-4 and GL-5
Glenn Kaufman asked what the difference is between GL-4 and GL-5.
The difference is friction modifiers. I can't remember which is
which offhand, (altho I think trannies use GL-4, don't take it to the
bank), but there's a difference in how they work.
Transmissions require a certain amount of friction to spin the sync
rings as the shifter moves from gear to gear. If the gear oil is TOO
slippery, the sync rings don't grab properly on the next gear as it
engages, and you get grinding gears. Therefore, running gear oil
which is too slippery is not good. Ergo, a gear oil with the same
weight as others, but a different designation: GL-x.
OTOH, most final drives (whether transaxle or differential) use gear
oil which should be more slippery. (Let's leave Positracks out of
this for all you old Detroiters, OK??) So although the viscosity
mayt be the same as tranny oil (90, 75-90, 75-95, and other flavors)
it has a designation as GL-y to note that it is not for use in
Synthetics make the distinction more important because they are
better lubricants than petro-lubes. Your dealer will have a
reference book to check whether your car's tranny and final drive
need Gl-4 , GL-5, or a different lube for each.
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