Rusted out head gaskets. Although, there's literally no such thing as a blown head gasket on a nailhead (I'm serious. NEVER seen one in all these years. Even with felpro which dump oil out the ends of the heads, even those don't blow.), if you have water in the oil on these engines, its one of two things, and thats it. the timing cover or timing cover gasket has failed, or the heads/block are cracked. that included cracked heads from valves seats. the original and other steel shim style head gaskets rust out with time. Seen it happen as quickly as 5 years, which is why I recommend composite gaskets. they are thicker, which we typically take into account when decking the block during a rebuild. we have had customers call and tell us the head gaskets we sold them were defective because the coolant passages were blocked off, so they punched holes in them. DO NOT DO THAT. Even with proper coolant ratio, by design, cavitation occurs from trapped air bubbles up against block off areas of head gasket, which essentially creates accelerated rust. running an overflow tank, does in fact help keep oxygen out of the cooling system. Flow of coolant under normal designed conditions, starts from timing cover, goes back to the end of the block. Comes up to cylinder head at very back of the engine before coming forward and reaching thermostat. The hottest the coolant gets is at the water manifold/right before it reaches the water manifold. If thermostat is not open, water then circulates back around through bypass nipple on water manifold, until thermostat opens at specified rated temp. That helps eliminate hot spots and keep cooling system from building pressure. The head gasket is responsible for forcing the water to go to the back of the engine. Once the head gasket rusts out, the water shortcuts through the engine and stops absorbing it's heat efficiently. It's one of the most commonly over looked reasons for overheating on these engines. If you're pulling the engine out of your car, even if just to repaint and detail, always change all the gaskets. It's silly not to.