Antifreeze can I mix? In short, NO.
IAT (Inorganic Additive Technology) – Bright Green, made from ethylene glycol with silicate and phosphates added to prevent corrosion. This is the oldest coolant blend and was used by all engine manufacturers into the late 1990's. The phosphates and silicates and work reasonably well with cast iron engine blocks and copper or aluminium radiators. IAT coolants must be flushed out and renewed every 2 years or 36,000 miles. If left in an engine beyond these limits, the inevitable formation of clogging solids could degrade a cooling system’s performance. Heater core blockages were not uncommon with this coolant years back if it was not regularly replaced. See link
OAT (Organic Acid Technology) – Organic Acids – made with a propylene glycol base and usually orange in colour but other colours are available such as dark green. This type of coolant contains no phosphates or silicates and is found in many engines built after 1994. Extended coolant life of up to 5 years or 150,000 miles is a benefit with these coolants.
HOAT (Hybrid OAT) – Silicates and organic acids – Yellow These coolant blends fall between the IAT and OAT types and are called Hybrid Organic Acid Technology (HOAT) products.
Si-OAT (Silicated HOAT) – Silicated HOAT is usually recognizable by its vibrant purple color. It uses silicates and organic acids to inhibit corrosive action in your engine. Its formula is free of nitrites, nitrates, phosphates, borates, amines and imidazole. Instead, it uses silicate, organic technology.
P-HOAT (Phosphated HOAT) – Phosphated HOAT uses phosphates and organic acids to inhibit the corrosion of your engine’s parts. The coolant is usually either pink or blue.
HOAT (Hybrid OAT, Phosphate-free) – Phosphate-free HOAT is usually turquoise in color. This NAP-free formula, made with ethylene glycol, contains organic and inorganic corrosion inhibitors to protect your engine. It does not contain phosphates, such as nitrite, nitrate and borate. It is also a low-silicate formula.