Datsun 120 y gear box oil

SOURCE: misfiring in 3rd and 4th gear

This sound like a timing issue, Check the points for proper gap, ck for proper timing, also ck the distributor shaft for looseness, or wabble, remove the cap and button and grab the shaft with your fingers and see if it moves back and forth, not in/out thats ok, but in a side to side type of movement. This can cause a missfire at hi rpm. 

SOURCE: Cant find where to fill

if it is a manual gearbox there should be a plug on the side of the gearbox,it could be either side, it is oil level as well as the filler plug,you will have to access it from under the vehicle,just screw it out & add oil until it runs out of the hole,then replace the plug.......hope this helps.....cheers.