I'd snug the bolts up a little on the valve cover gaskets and continue to run the Delo 400 and change it just like before. The 4L80E, I don't think I would switch to synthetic on it, just the dexron that was recommended for it when new, if you can still find it, but convention trans. fluid (Dexron) is what I'd use. The differential, yeah, go with the synthetic, either GM's or Mobil 1 or Amsoil.