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I used Pennzoil Synchromesh in my 5 speed, nv3500. The Pennzoil will do the job perfectly, no need to spend the extra on RP or Amsoil, in my opinion anyway.
All a matter of opinion, Any thing is better then the old crap that came from the factory!
I personally like amsoil simply because I have had great success with it. Plus I run synthetic everything in my truck. But its not required obviously.