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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.

Al Powell                           Voice:  409/845-2807
107 Reed McDonald Bldg.             Fax:    409/862-1202
College Station, TX 77843           Email:  a-powell1@tamu.edu 
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"Baseball is 90 percent mental.  The other half is physical"...
...Yogi Berra.