Gearbox oil does not need to be replaced according to Volkswagen specifications. It is sufficient to top up the oil every 10,000 km until it overflows. The only thing Volkswagen prescribes is to change the gearbox oil 1,000 km after an overhaul, thus flushing the gearbox and ridding it of gasket remnants and other dirt left over from the overhaul.
We advise you not to fill up the gearbox, but to change the oil immediately. It doesn't cost much, and fresh oil is always preferable to oil that has been in use for many years.
The plug mentioned here has an additional function on the Bay Window Bus and VW Thing models fitted with a gearbox reduction: it serves both as a filler cap and as a drain plug. Volkswagen also recommends refueling only, but even then it's advisable to change the oil, as it's not known how long it's been in the vehicle.
If the filler cap and drain plug are not unscrewed with the right tools (see the "options" tab), or if the hexagonal key is pressed in while everything is still covered with mud, this can damage the cap inside, causing the hexagonal key to round off the hole. In this case, you need to replace the plug.
The red in the thread of the new filler cap is a sealant which ensures that the cap is 100% leakproof. If you unscrew the cap at a later date, there's no need to re-apply sealant to the thread.
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