My 89 Syncro has knocking lifters at times.  The longer it sits, the
louder they knock, especially if it has been sitting with the front end
higher than the rear.   Rarely if the nose has been lower than the rear.
 I have owned it about 13 months.  It came with the knock.  PO or his
mechanic used the wrong filter.  Changed to the right filters, only
minor improvement.  Tried the Marvel Mystery Oil bit on the next change,
no improvement.  Again, no improvement. All of this with 20W50 oil.
Recently switched to Mobil 1 15W50 and the problem has almost disappeared.

On the other hand, my 84 (original owner), has only had knocking lifters
once, after sitting unused with the nose higher for about 3 months.
 Quieted down in about 3 minutes and haven't heard a sound since then.
 Correct filter and oil in recent years.

Sam Walters
Baltimore