I thought the oil coolers failed because the silica found inside the gold coolant would overheat and gum up the oil cooler leading to everything you mentioned, not the casting sand. I switched to the red ELC after a flush and haven't had any issues since.
The biggest thing is the casting sand left over in the bock, that and rust. these came from the factory with Gold coolant and that's all I run in them.
How exactly do you then clean the coolant lines and engine chamber where the coolant-oil mixture had already passed through and got deposited at every nook and corner?
@AbhishekKumar1902 Take the heater care line off next to the alternator, open radiator cap and coolant drain plug, start engine. Put a garden hose on full blast into the heater line you removed. Run till water comes out clean. I've run a cup of purple power heavy-duty cleaner in the cooling system with water for a good long road test and then flush again. After all that, replace the degas bottle and fill with fresh coolant. Over the next few weeks of driving, the remaining oil will rise to the degas bottle. Replace the bottle again. (You can buy bottles on Amazon for under $50)
