Internal leaks are when the fluids enter another circuit and may result in changes to the coolant or oil. The former may be the presence of foam (caused by hydrocarbons) in the coolant expansion tank. Coolant leaking into the oil system may result in a mayonnaise, or milkshake-like, substance in the oil, often to be seen on the dipstick, or oil filler cap.
Internal breaches refer to instances where fluids escape into another circuit, which can have consequences for the coolant or oil. This might be identified by foam (caused by hydrocarbons) in the coolant reservoir or coolant leaking into the oil system, leading to a substance resembling mayonnaise or milkshake appearing on the dipstick or oil filler cap.