Opel Corsa B 1.2 1996 gearbox

HE CORSA 1.2 Gasoline 1996, my problem is in the gearbox. In the upper part of the right side, where the gear lever is in speed is a local oil breathtaker before the rubber came out a little oil now stopped out as I can to replenish and what reference. On the top it has a plastic cover of a plug. Can I restore the level unraveling the boxwood? Please help me.

If I do not remember the oil level of that box, it is measured by drawing a plug that is near the trip on the driver's side, take it out and if oil comes out it is complete