Was the o-ring broken when you pulled it out? A little hint now that it's out.....I've had problems with "other than stock" thermostat seals/gaskets. As far as I've ever found, the stealer is the only one that has that o-ring, and it only comes with the thermostat (~$30). You apparently can't purchase it separately. So, I've tried numerous different gaskets, and all eventually seep or leak, except for the Fel-Pro P/N 35840. I've also tried gaskets with and without RTV, as well as RTV on an old stock o-ring. Some lasted a while, but eventually started seeping. I ended up going back to the stock setup, but a different thermostat housing (other than the stamped metal) might also resolve the problem, using a gasket. Let us know what you end up doing. I think the seepage happens with a gasket because of the aluminum timing cover and the metal thermostat housing.
06 GCO, Dk Khaki, QDII, Flshing Hi 3rd Bk Lt, MDS "ON" Lt, Airtab Vortex Gens, Debadged, Fumoto Drain Valve, BT Catch Can, Chopped EGR Tube, SRT8 CAI, K&N Filter, Sonnax Trans Line Press Booster, SC Tuner W/Adv Trans Tune App, DS Tuner, Cust tubular hdrs w/3 in Magnaflow/Xlerator AP Exh Sys, Bilsteins, SRT Springs/Sways, Cust Eng Cvr, SRT8 frt/rear bumpers, FRI Sidewinder Cam/6.1 valvetrain, SLP 25% UD Pulley, BBK true 90mm T-Body, SOS Ign Coils w/.060 spark plug gaps, SRT Reps with 265/45&295/40X20's.