How much gearbox oil must you put into a 1978 VW beetle?

There is no gearbox (transmission) in a 1978 VW Beetle. The Beetle makes use of a transaxle, which contains the transmission and differential in a single unit. The proper way to add oil to the transaxle is to first open the drain plug on the bottom of the transaxle and allow all of the old oil to drain out. Once the old oil has been drained, replace the drain plug and refill the transaxle with new oil through the fill plug on the side of the transaxle. The amount of oil required will depend on the specific model of Beetle, so it is best to consult the owner's manual for the exact amount.