That's how they all are these days and they stay just fine. I did this same thing minus the drain plug on my 03 this summer along with both diffs, the transfer case and the torque converter. The fluid in the trans/converter was pretty new since the dealer I got it from serviced it but the stuff in the transfer case was black and the stuff in the diffs... shudder... I will say the 4x4 works 1000% better since I put fresh fluid in there. Did it all prior to a 1200 mile round trip which really isn't that much but did not want to have an issue 6 hrs from home.