Question
my bushes are 20 yrs old I just noticed one has grey scales over the main branches and the other branches are dying. What is it and can it be saved

Answer
If you lilac is heavily infested with scales, it can kill the plant.  Branches that are heavily infested should be pruned out.The best method for control is to spray when the plants are dormant (winter) using a  dormant oil spray.  This is done before bud break when the plants are dry.  You will also want to spray nearby shrubs because they too may be affected.  It is best to apply at least 48 hours prior to a frost since plant injury can occur.  

It sounds like your plants have a heavy infestation at this time.  Since you are past the blooming stage, you should use a "strong" insecticide such as malathion on the plants.  This product will kill the immature scales in the crawling stage.  There is not much you can do to get the adults at this point.  You may need to treat the plants more than once.  If another application is needed, you can probably switch to a more "mild" product such as insecticidal soap to kill crawlers.

Good luck!