These engines are known for warp/cracking. It doesn't take much oil to really make coolant look nasty. The engine could be picking up oil in an intake runner and pumping it into the cooling system thru a crack/warp or blown/leaking head gasket. Doesn't take much..
After overheat these heads are usually junk. Getting hard to find good used ones because of that fact. I know NONE of these, and for that matter, the 3.0-4.0 are available in my local area.. Aftermkt stuff is available tho.