there is no issue using locktight on nuts with a nylon insert. It will work, and well. What they mean when they say that is that, say you had an all nylon nut and opted to use locktight...it won't be effective. It's not going to blow up or anything. There is plenty of metal to grab hold of on a standard locknut. Just get some $1 thread locker at harbor freight, apply lightly, and be done with it.