Gear oils are complex mixtures of a base oil (mineral, synthetic, or a blend) and a sophisticated additive package. This package includes:

Extreme Pressure (EP) Additives: The Most Critical Component. These are typically compounds containing Sulfur (S) and Phosphorus (P). They react with metal surfaces under high pressure and temperature to form a protective sacrificial layer that prevents welding and scuffing.

Anti-Wear (AW) Additives: Such as Zinc Dialkyldithiophosphate (ZDDP), which form a protective film on metal parts.

Friction Modifiers: To improve efficiency and reduce heat.

Rust and Corrosion Inhibitors: To protect against moisture.

Oxidation Inhibitors: To extend the oil's life by preventing breakdown at high temperatures.

Pour Point Depressants: To allow flow at low temperatures.

Foam Inhibitors: To prevent foaming, which reduces lubrication and causes overheating.

Viscosity Index Improvers: (In multi-grade oils like 75W-90) to maintain viscosity across a range of temperatures.

The Problem with Blending

When you mix two different gear oils, you are mixing two different chemical additive packages. They can interact in negative ways:

Additive Antagonism: The chemicals can react with each other instead of with the metal surface. This can cause:

Precipitation: Solids can form and drop out of the solution, clogging filters and leaving surfaces unprotected.

Gel Formation: The mixture can turn into a thick, jelly-like substance, starving the gearbox of lubrication.

Foaming: The foam inhibitors can be neutralized, leading to excessive foaming and aeration.

Neutralization of EP Properties: The critical sulfur-phosphorus EP film may not form properly, leading to immediate gear damage under load.

So, When is Blending Done? And How?

Blending is done on an industrial scale by lubricant chemists, not by end-users in a garage. The "special additive" in this context is scientific analysis and testing.

The process for creating a compatible blend involves:

Compatibility Testing: The two candidate oils are mixed in a 50:50 ratio and heated. They are observed for signs of precipitation, haze, or gel formation.

Performance Testing: The blend is tested in a rig (like a Four-Ball EP Test or FZG Test) to ensure the AW/EP properties have not been degraded.

Filterability Testing: Ensuring any reaction byproducts do not clog filters.

Practical Advice for the User

Best Practice: Do not mix gear oils. Always use the same type and brand specified by the equipment manufacturer. If you need to top off, use the same product.

If You MUST Mix (e.g., in an emergency):

The safest bet is to mix oils from the same brand and with the same API service classification (e.g., API GL-5) and viscosity (e.g., 80W-90). Even this is not guaranteed, but it's much safer.

Mixing synthetic with conventional is generally acceptable if they are from the same manufacturer and have the same service classification, as their additive packages are likely compatible.

Never mix oils with different API service classifications (e.g., do NOT mix a GL-4 with a GL-5). Their EP additive levels are drastically different and chemically incompatible.

After an Emergency Mix: The correct procedure is to consider the fluid contaminated. You should flush the system and perform a complete oil change with the accurate, uniform oil at the earliest possible opportunity.