the only way is to shoot some in bottom plug til comes out the top hole,be sure to use the proper gear oil,you may have electric shift and it uses a different oil than what is used in newer engines
pull the vent plug and see if anything it at that level. If it's been a while needs changing anyway. Like it's been said pull both plugs and let it drain. Look at the oil for water infiltration and feel it for metal. After it's drained fill it from the bottom until it comes out the top. Put the top plug in while still holding the pump to the bottom then quickly put the bottom one in. Do make sure you use the right lube as has been pointed out.
