Welcome to the forums, there's a lot of helpful info here in case you ever have problems! As far as the plugs go, it is true that these motors don't really enjoy platinum plugs, which is probably why you're getting worse MPGs. Iridium is the factory plug, and IIRC the AC Delco part number is 41-101. LIM gaskets are probably at the point of needing to be replaced, but you'd wanna check for leaks around the LIM valley by the valve covers, and on the passenger side of the LIM.