Son owned an 05 Explorer 4.6L, spec'd 5w20, quite certain 05 Mountaineer 4.6L does too. OLM light went off every ~5k, not a true OLM. At 5k OCI's, most any 5w20 dino will handle that. MC Syn Blend is excellent but getting relatively pricey. I'd stick with any major's 5w20, QS, PYB, VWB, GTX, or MS5K. Add a MC FL820S oil filter and you should be good to go.
If you want to use a synthetic, QSUD is the best major's everyday value at Wally.
Owner's manual pdf can be found in the link. Pick an Owners Guide edition on the right side menu. http://owner.ford.com/servlet/ContentSer...del=Mountaineer