I don't know if GM techs use this method, but Im gonna tell you how Honda techs do it. It may work, but def ask to speak to a local GM tech before actually doing this.
Drain all oil from the drain plug first, remove oil pan and gasket. Remove spark plugs and Coil on plugs. Remove valve cover and valve cover gasket. Now here is where its tricky, there are 3 ways I have seen it done. Honda puts ATF +4 transmission oil on top of valve train, sits there for 24 hours (ATF +4 is also a high detergent, breaking down oil sludge and deposits). Make sure you also have a 10 gallon bucket to catch the trans oil. Flush it down with new oil. Run car for 15 minutes while maintaining RPM's @ 1200-1400. Drain oil with new oil and filter.
That's Honda's way of cleaning out sludge, Nissans is same method, but with Ethos Oil (unless you have a VQ37HR engine which requires Ethos Oil). Ask someone who has done this at a Chevy Dealer first before any attempts.
2009 Cobalt LS XFE U74
Mods: K&N Typhoon Intake, Option Cat-Back Exhaust, Cosmo Short shifter, Maxxim Ahead 17X7's, MPx shorty antenna, FE5 Strut/shocks, Tein S Techs, Moog endlinks, Moog spring seaters, ZZP shorty header, LSJ downpipe, LNF front grille