The data in the following article is collected by me analyzing articles on the Internet, I am not a mechanical specialist, I recommend you also verify it yourself.
VW's official 2004 guideline (four-wheel drive 02G) states:
- 1.0 liter “G50 75W90 (synthetic oil)” for the differential
- 2.7 liters of “G51 75W90 (synthetic oil)” for the transmission
Unfortunately, VW's internal specifications do not relate to international specifications. However, it can be assumed from gear oil manufacturers that VW's internal G50 standard is close to the international API GL4 standard. VW G51 is probably similar to the unofficial classification according to API GL4+ or GL4/GL5.
The differential manufacturer “Steyr-Fahrzeugtechnik” or its service “S-Tec” writes 1.3 liters (pre-fill) according to the specification “TL52726”, which corresponds to the API-GL4 class.
However, our recommendation for the differential is as follows:
For heavily loaded hypoid teeth, where the crown and pinion are in contact only over a small area (extreme surface pressure), the GL5 standard is optimal. For this purpose, the base oil contains numerous additives. For this purpose, the base oil is enriched with sulfur and phosphorus additives. During driving, the surface of the sides of the teeth becomes phosphorescent under pressure and heat. This phosphorescence acts as an additional protective layer on particularly stressed surfaces and reduces wear.
Manual transmission : 75W90-GL4+
Although the same hypoid gear pattern is also found in manual transmissions, the gear synchronization rings must be considered here. Because GL5 lubricates so well, it can cause the synchromesh rings in the gearbox to slip and partially lose function – which can cause gear rasping and wear during gear changes.
In addition, GL5 oil additives can attack the non-ferrous metal component (molybdenum coating) of synchronizer rings.
But there are also oils with the unofficial name GL4 / GL5 (GL4+), which balance ideal lubrication of the hypoid gear and residual friction for synchronization. This unofficial standard is our recommendation for a manual transmission and should also be compatible with the VW G51 standard.
Gear oils are classified by the American Petroleum Institute (API) based on GL rating. The higher the oil's GL rating, the more pressure can be maintained without metal-to-metal contact between transmission components.
Defines the type of performance characteristics of bevel gears and hypoid gears in automotive axles operating at moderate speeds and loads. Gear oils for API GL-4 are typical representatives in transverse-mounted transmissions today.
Determines the type of performance characteristics of hypoids in automotive axles under conditions of high speed and/or low speed and high torque. Gear oils conforming to API GL-5 are currently best used in differentials.