I am still running the summer blend at 2 oz to 20 (earlier thread I may have listed worng) but I find I have been getting about 1 - 1.5 mpg better (overhead stay between 26.5 * 27.2) I have started driving more consistanly also...easier takeoffs, coast to lights if in city and staying at or below 2k rpm's.
I started with the 1 oz / 20 gal and then moved up thinking the same thing you were, I have not jumped up any further since I am happy right now. Also I do believe the XPD was a experimental when that test was done, if you see the actual report I 'think' that is the way it was listed.
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2006 2wd 2500 - CFM intake - Baldwin B50 oil bypass -Aero 4040XL Muffler - K&N Air - Cool Blue Hose - Delvac Rotella oil, everything else Amsoil - Alpine 505 / Blackbird nav / XM (XM 202 Sirius 197 - O&A RnF) , Optilube Summer blend