I don't oil the spring at all, though a little oil does run down there when you stand up the rifle with a wet BCG. I don't see any reason to go out of my way to lube the receiver extension. Some folks gunk it up trying to dampen the "sprong" noise though.
As far as the polymer lower needing the buffer spring greased, that's a new one on me. Seems to me it wouldn't matter since the receiver extension, buffer and spring still are metal.