I have always run synthetic oil + Muscle MT-10 oil treatment in all of my vehicles, including past 4.0's I've owned. (Muscles does not have all of the crap like teflon that can build up in the engine.)
After 1000k on my JK I changed the oil and went with the synthetic + Muscle. (I know, I know, the Jeep manuaul "strongly discourages" the use of additives.) I have to say I've noticed an average of 2 MPG increase (over 1000k since I changed the oil). This stuff is great . . .
However, I was wondering if I should add a small amount of Muscle to the transfer case and differentials . . . ? The company that makes Muscles says you can add it to anything that takes oil, but would like to know if any of you have experince with oil additives in these areas and if it makes any difference.