Selecting the right gear oil

We are frequently asked which gear oil we recommend for the T3 Syncro. The official VW guideline from the ‘80s speaks of 75W90-GL4 for differential & transmission.

 But our recommendation is different:

Differential: 75W90-GL5

For the highly stressed hypoid teeth, in which the crown and pinion touch only along a small surface area (extreme surface pressure), the GL5 standard is the optimum. For this purpose, the base oil is provided with numerous additives.

 Manual transmission: 75W90-GL4+

Although the same hypoid gear pattern is also present in the manual transmission, consideration must be given here to the synchronizer rings of the gear wheels. Because GL5 lubricates so well, it can cause the synchronizer rings in the transmission to slip and partly lose their function – which can cause gears grinding and wear to occur when changing gear.
In addition, the additives of the GL5 oils can attack the non-ferrous metal component (Molybdenum coating) of the synchronizer rings.

But there are also oils with the unofficial name GL4 / GL5 (GL4 +), which manage the balancing act between the hypoid gearing’s ideal lubrication and a residual friction for synchronization.


API ratings

Gearbox oils are classified by the American Petroleum Institute (API) using GL ratings. The higher an oil’s GL-rating, the more pressure can be sustained without any metal-to-metal contact taking place between transmission components.

Designates the type of service characteristic of spiral-bevel and hypoid gears in automotive axles operated under moderate speeds and loads. Gear oils to API GL-4 are today typical representatives in transaxle transversely mounted gearboxes.

Designates the type of service characteristic of hypoids in automotive axles under high-speed and/or low-speed, high-torque conditions. Gear oils according to API GL-5 are today preferably used in differentials.



After a VC fails closed, it then fails open.

We suspected this issue for a long time, but now we found evidence to support our theory.
But let’s tell the story from the beginning.
A customer brought us his old Viscous Coupling (VC) in an unknown condition to our workshop, After the first visual inspection, I suggested to the customer that we could actually save the time on the test-rig, because fragments of the upper X-ring were already hanging out of the lubrication hole of the inner hub (see attached photo). I assured him that his VC had failed open (no drive) and would only reach about 50 nm friction moment.


But when the test-run started, we were all very surprised! The Viscous Coupling was not failed open, but closed (no slip at all)! It had sucked oil inside.

After testing all VCs from my own stock with obvious X-ring damage, I found one more VC in a similar condition.

This Visco clutch shows an advanced stage of failing open. The lowered moment indicates that some silicone oil has already leaked out and the Visco will soon completely be failed open.


Our Theory:

The constant hump pressure (>10 bar) in the oil-sucked Viscous Coupling causes the rubber of the X-ring to creep and slowly be pushed into the gap. As a result, the X-ring can no longer create a seal.

To cut a long story short: After a VC fails closed, it then fails open.