I have seen a few threads about oil being used in the 6.0 engine & i just have a few questions or concerns. what should i be running? being from Illinois, you all can assume the temperature extremes here. ive been using rotella 15w-40 on my 2006 since i got it used at 90000. its now at 130000 & i just recently switched to Shaeffers 15w-40? any thoughts or opinions?