Don't mix the green, silicate-type antifreeze with orange antifreeze. The organic acids in orange types will cause separation of silicates in the green type, which greatly reduces corrosion protection.
Car Radiator Coolants InformationMixing the antifreeze types should not have been the problem, it still cools, however it allows corrosion to occur, but that takes a long time to happen.
If you were constantly having to add coolant with no visible sign of a water leak, and the oil level was over full when you checked it (coolant leaking to crankcase). Water in the crankcase oil results in a frozen engine. The mechanic should have checked for frozen engine before he ordered the heads!
