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.

GL-4: 
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.

GL-5:
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.