I can't believe that the gear box you've shown in your thread is lubricated with grease. There must be some sort of error. If your sure theres no filler port. I would drill and tap the cover to a 3/4" pipe thread about 1/3 the way down from the top of the cover. Install a street elbow with a threaded plug in the elbow with it pointing up with the cover installed. I'd use 80/140 grade gear lube. I'd put a quart in the gear case loosely install the plug and turn the spindle on and look for leaks. If none are found I think I would put about a total of 3 quarts of the gear lube in the gear case and call it good. checking for leaks and fixing as required.