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.
02 2500HD LT D/A SB CC 4X4 BLACK, Westin stainless nerf bars, BW GN Hitch,Racor 60S post oem fuel filter, Oil Guard bypass engine oil Filter. All synthetic fluids. Kennedy boost valve, edge, Modified air intake,EGT & Boost digital gauge,TransferFlow combo fuel & tool box, Air Lift Suspesion Bags Rear & compressor with remote, Bilsteins front & rear, Frontier front replacement bumper.