Transfer case oil.
If you want a copy of your owners manual wherever you are, you can download one for free at www.motorcraftservice.com.
I've had the fill plug out about a month ago to check the level, so it should be no problem. Decided it was a good time to change all the oils in the truck, since I haven't done it since I got the truck. And its been long enough since they were done before(truck was my dads).








