The moving parts in an average gearbox are lubricated by approximately 2.5 litres of oil. If that level drops to below two litres, then gear changing may become difficult. Eventually, the gearbox may become damaged due to friction and may ultimately seize up completely. Look out for fresh oil stains under the gearbox of a parked vehicle, as these can be indicative of a gearbox oil leak. If it turns out that your gearbox is leaking oil, then it is important that you check and, if necessary, replace the seals as soon as possible.