Some Definitions
Transmission
An automotive assembly of gears with a number of selectable ratios, used to match the engine's rpm and torque to differing vehicle requirements. The transmission is the assembly by which power is transmitted from the engine to a driving axle. Also called a gearbox.
Differential
A gear system that allows one wheel to rotate faster than the other while providing equal power to each wheel, as necessary, when turning or cornering. It is located in the transaxle of a rear engine, rear-wheel drive vehicle.
Transaxle
What most people refer to an the "transmission" in a VW is actually a "transaxle." A transaxle is used in front-wheel drive and rear-engine, rear-wheel drive vehicles such as the aircooled Volkswagen. Transaxles incorporate both a transmission and a differential into a single unit.
From the Haynes Manual - The manual transaxle is a compact, lightweight aluminum alloy split case (two piece) housing (early models) or a single-piece housing (later models) containing both the gearbox and differential assemblies. Early split-case units have four speeds, with a non-synchromesh first gear; later single-piece units are full synchro four-speed designs.
Syncronizers
The VW transmission relies on brass syncronizer rings to allow matching of gear speed to transmission input shaft or pinion shaft speed to facilitate grind-free shifting. All forward VW gears have their own syncronizer. Reverse does not have a syncronizer. This brass syncronizer ring is a kind of brake pad acting against the gear itself. Each "brake pad" syncronizer has to stop it's gear from turning every time a different gear is selected. When a car's driver moves the shift lever faster from gear to gear, the syncronizer ring has to stop it's gear from turning in less time than if the shift were made slower. Sometimes a gear change is made faster than what the brass syncronizer can physically handle, and the gears grind together before they mesh, often causing only very slight damage to the gear teeth as the gears are extremely hard. (Reverse gears are the exception here, they are soft.) So, it is possible to overwhelm the syncronizer's ability to perform it's function as the speed of shifting is increased.
Gearbox Oil
Using a 17mm Allen/Hex head tool, remove the plug and insert your finger in the hole.....the oil level should reach the edge of the opening and be on your finger tip.
The vehicle must be as close to level as possible to check the oil accurately.
When changing the oil, remove the two plugs under the transmission. After all the oil has drained, thoroughly clean the plugs and reinstall them and note how much debris there is. Fill the transmission using SAE 80W90W Hypoid gear oil (GLS4). The transmission’s oil capacity is approximately 6 to 7 U.S. pints (around 3 litres).
Fill the transmission slowly otherwise if you fill it too fast the oil will overflow and give you the false impression that it is full when its really not. After you have reached the correct level, clean and reinstall the filler plug.
I have attached a few good links to other pages in order to provide you with some insight into choosing the correct gearbox oil.
RICHARD ATWELLS'S SITE - RATWELL
The above information is provided from the the following resource: