What I am thinking is that
if the yelllow metal problem doesn't actually exhibit itself in 094's because of the sintered synchro cone spec (and VW did tend to over specify materials) than we could all reap the benefits of GL-5 i.e. it is far better at keeping the steel components apart at elevated temperatures and pressures.
Potentially good news for everyone.
I don't have a choice really, there isn't a GL-4 LSD oil and from what I have read, you can't have a GL-4/GL-5 because the oil performance specification is so different, as per the Corvair article at the start.
History:
The gear oils of a few decades ago had lead additives that were effective at wear reduction, but not
very good for the environment. A long time ago they began to be replaced by gear oils with a
phosphorous additive (in itself a decent anti-wear additive) with active sulfur to grip hold of the gears
and create a very solid sacrificial layer of material that could be worn off, thereby protecting the gear
surface. Eventually it was discovered that the active sulfur was causing corrosion of brass and other
soft metals used in differentials and transmissions.
Somewhere around 20 years ago a deactivated or buffered sulfur was developed that would react with
the phosphorous to create the protective/sacrificial layer in the conditions created in the gear boxes
(temperature and pressure) without being corrosive to the brass, copper, etc. This additive system is
used in most gear oils today.
In normal operation, the sulfur/phosphorous additive forms a black sacrificial coating on the gears and
anything it touches with a little pressure and temperature. As the gears turn, instead of wearing, the
sacrificial coating of additives is pealed off or worn off. This is normal and acceptable in all steel gears.
But when one or more of the surfaces is brass or another soft metal, the sacrificial coating is stronger
than the base metal, and instead of just peeling off, it takes with it a few microns of brass that it is
bound to.
A normal GL-4 gear oil of any given viscosity has about ½ of the level of sulfur/phosphorous additive
that would be in the GL-5 product, so the bond is not as strong, and therefore can be peeled off without
peeling a layer of brass (or less brass). This means that the GL-4 product provides a little less extreme
The Difference between GL-4 and GL-5 gear oils 2 of 10
pressure protection, so in the differential of a high powered car, it would not be the ideal product in the
differential. To understand this need we should be aware of the fact that the differential is where the
final torque is applied to the wheels (in most applications).
But in the transmission, we should consider two factors:
• Do to the fact that the differential applies the final torque, normally we do not need the full EP
protection in the transmission where less torque (about 30%) is applied.
• We need to be able to break the EP protection to stop the spinning of the gears long enough to
mesh them or synchronize them.
When we use a GL-5 product in a transmission that requires GL-4, we normally find 2 to 4 times as
much copper in the used oil as we would with a GL-4 product. Eventually the synchronizers wear to
the point that they no longer make contact with the other half of the cone, bottoming out before
stopping the opposing gear.
I think we need Aidan to tell us what was fitted from new (as far as can be certain) and what is currently supplied by Classic Parts.
1985 Oettinger 3.2 Caravelle RHD syncro twin slider. SA Microbus bumpers, duplex winch system, ARC 7X15 period alloys
I will have to slap it in for now and change it when I have found an alternative.
, but modern spec of 75/90 seems to translate as slightly easier shift when cold those first few shifts of the day while still giving the same 90 protection when hot - most modern vehicles seem to use 75/90 so that is what is more commonly available now and most of the 75/90 will say on the label they suitable replacement for 75/80 75/85 as used in some cars now
