Use 75W90 gear lube. Sorry I have never worked on a 7-1/4 rear so I don't know the size of the plug. The leaking seal your referring to is the pinion seal. The only hard part to changing it is getting the pinion nut loose and tight again as the yoke will want to turn when you do it because it's torqued to 240 ft. lbs. They make a yoke holder tool that makes the job a lot easier. Another thing to note is the seal isn't always the reason for a leak. A lot of the time a groove wears in the yoke where the seal rides so a new seal won't stop the leak. If it's not dripping or making a mess I wouldn't worry about it.