GL 4 and GL 5 refer to different API performance categories.

VW suggests a GL 4 for that transmission. Some people think that since a
GL 5 is an oil designed to take even more load, it should be better
right? Wrong.

In this case you must examine the additive package closely. Most
products use sulfur based EP additives. Sulfur is a great EP additive,
but is corrosive ( thats actually part of how it works, but thats a
different topic ). On gears this corrosive nature is okay. So GL5 oils
are mainly used for differentials. Manual transmissions have synchro's,
and brass is not as tolerant to the corrosive nature of the sulfur.

So trannies use GL4's.

http://www.lubrizol.com/referencelibrary/readyreference/15-gears/gearclass.htm

You can read about it above and here below:

http://www.lubrizol.com/referencelibrary/readyreference/15-gears/geartext.htm

You can chuck this bit of logic out the window though if the oil your
using is based on a boron based additive.

Chevron makes a few gear oils that use boron...its a GL-4, GL-5, MT1 (
the highest category), but sicne it uses boron, your synchro's are even
safer. This kind of additive package is the absolute best to use. It
produces less wear then sulphur based additive packages. However chevron
only uses it in mineral oil based oils. However, even in the mineral
based version it meets all the extended oil drain specs that only
synthetic gear oils do. Consider it. Unless you live somewhere really
cold. Nissan uses them for racing...and I've seen pictures of the gear
boxes torn down. Less wear then when they used Mobil synthetic products!
If only Chevron made a synthetic version with this additive
package...but they don't because even in mineral form it still beats the
synthetic competition. ( except in super cold climates - alaska, NWT,
yukon etc ).

Otherwise, Redline MTL and MT-90 are the only synthetic GL 4 oils on the
market.

Its cold where I am, ( -20c outside last night, and been that way for
the last few weeks ). So I went the redline route. If I lived somewhere
warmer though...I would use the chevron product. Chevron delo trans
fluid ESI ( esi is the extended oil drain time oils they market ). Just
ask the dealer if its the product based on boron. You'll know because
its the only mineral based transmission oil that also meets the EATON
750,000 mile oil drain time spec.

On Sat, 29 Jan 2000 18:13:12 -0500
Blue Ridge CycleWorks <brbikes@RICA.NET> wrote:

> The other day I recall a reference that made me believe that I should use
> GL4 but not GL5 in the tranny (1981 Westy, 4 speed manual) Why not? What
> does the numbering refer to? I will soon be switching to synthetic and only
> want to do it once.
> Thanks in advance.
> Myron Lind
> 81 Westy

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Dominique Cormann                              Email: kozmik@home.com
Homepage: http://kozmik.guelph.on.ca
Diesel page: http://kozmik.guelph.on.ca/gtdproject
84 Rabbit D - daily driver
84 Rabbit GTD - work'in on it