The concentrate was hard to find here too. I ended up giving my tech 6 gal. of 50/50 for installation after stud and hg job. Only tested out at -7 deg f. I ended up having to drive 45 min. to the nearest cat shop and got concentrate there. $36 a gal. for the cat stuff. Drained out 2 gallons and added the concentrate and now the mixture is perfect. Test's out at -36 deg. f. The heat transfer is awsome now. EOT is only 3 deg. higher than ECT driving around town. The highest differential I've seen so far is 7 deg. driving at 80 mph.
As a side note, I just changed my oil to valvoline premium blue extreme 5w40 syn. and added 2 bottles of rev-x and my engine fires up great and idles smooth and runs like a top! Could not be happier with my truck than I am right now.