Last month I changed my oil from reg to synthetic. Here recently, my car has been running hot so I've been topping off the coolant/antifreeze in radiator/reserve. The problem is only the reserve keeps getting low. I switched a couple of days ago to water in the reserve (coolant was getting expensive) and now I have yellow crap in my oil/oil cap. !! Is this a blown head gasket or just moisture from switching coolant to water in reserve??