Ford Explorer recent tranny problem

I recently had trouble with my 1994 ford explorer with 195.000 miles also V6 4.0 liter engine.I went to auto zone to have a pcm check the manager said he could only do it to a 1996 and up not a 1994 I thought this to be a little strange. Anyway I was told by a old mechanic who saw black smoke comming out of the tailpipe. He told me to put stop squeal in when I change my oil that will stop the smoke from comming out of the tailpipe