I see you checked the oil, but is it the correct weight....5W-20? Have you cleared the DTC? Has it returned? Do you regularly change your oil, or do you let it slide way past the change intervals?
The No. 1 cylinder is an MDS cylinder, and there have been reports of the MDS lifter actuating pin(s) failing and causing an engage/disengage problem. This usually causes an initial intermittant problem that progresses into a continuous problem. There have been some writeups, with pictures, on at least one of these forums. If you can't come up with anything conclusive, you may want to do a search and see if you can come up with one of them. IIRC, one that had the problem was going ~10,000K between oil changes. Did that cause it???? Who knows? Guess it could keep the pin(s) from moving as freely as the should, especially if running dino. oil that long.
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.