Hyundai Repair: Elantra running warm, 2003 hyundai elantra, hyundai elantra

Question
2003 Hyundai Elantra ran great for 400 miles after head resurfacing, valve job,new gaskets and head bolts..  Engine began running warm and mixed oil in water.  We cleaned the system, replaced the thermostat and put sealant in the cooling system designed for aluminum heads.  Engine runs normal for about 15 miles-cooling fans are operational- them temp goes about 50% over half way mark on temp gauge and holds running warm.  What should we do next?

Answer
If you were mixing coolant and oil, you have a serious problem.  Running about halfway between 1/2 and hot doesn't strike me as an issue, though.

To start diagnosis, I'd recommend starting with retorquing the head bolts and checking the radiator for clogging.  If you find the head bolts are all okay, I'd recommend removing the cylinder head so the head and block surfaces can be checked for warpage.  I've never seen a Hyundai with a head gasket issue that didn't also need a cylinder head due to excess warpage.