Front Diff fill/drain plug

[busta]

So I had my truck in a few days ago to get the front diff oil changed and then last night while i was getting my torsion bars cranked up by a buddy of mine he noticed i had a little (very small) leak coming from the drain plug...he got to looking at it and saw that there was a neoprene washer on the drain plug and there is also one on the fill plug...he told me that the plugs didnt need to have those washers on them because the bolt is machined so that you dont need an extra washer on it...any advice would be appreciated

 

2000 GMC Sierra 4x4

[GM-Tech]

The only drain/fill plugs I know of that don't have some sort of sealing washer are the type that are pipe thread and seal by thread interference.