IAG Air / Oil Separator (AOS) For 08-14 Subaru WRX, 08-21 STI, 09-13 FXT, 05-09 LGT & OBXT

Part Details:

The IAG Air / Oil Separator (AOS) is designed to improve crankcase ventilation and reduce oil vapor entering the intake system on a wide range of turbocharged Subaru platforms. By separating oil from blow-by gases before they are recirculated, this system helps keep intake components cleaner, reduce oil consumption, and support long-term engine reliability.

Unlike traditional catch cans that require manual draining, the IAG AOS uses a continuous oil return design that routes separated oil back into the engine. This closed-loop configuration maintains factory-style crankcase ventilation behavior while improving oil control under boost, making it suitable for both daily-driven vehicles and performance builds.

How the IAG AOS Helps Protect Your Engine

Turbocharged Subaru engines generate crankcase pressure that carries oil vapor through the PCV and ventilation system. Without proper separation, this oil vapor is drawn into the intake tract, where it can lower effective fuel octane, increase detonation risk, and contribute to carbon buildup in the intercooler and intake system.

The IAG AOS separates oil from crankcase gases before they re-enter the intake, returning the oil to the engine while allowing cleaner air to be recirculated. This improves combustion consistency and helps maintain engine health over time.

Low-Maintenance, Street-Friendly Design

Because the IAG AOS continuously returns oil to the engine, there is no need for routine draining or maintenance like with traditional catch cans. The closed-loop design preserves OEM-style ventilation behavior, making this AOS a practical upgrade for street-driven vehicles as well as higher-performance applications.

Street Series vs Competition Series

IAG offers both Street Series and Competition Series AOS options depending on vehicle use and power level.

• Street Series AOS: Retains closed-loop PCV operation and is suitable for street-driven vehicles, including those running E85 or other ethanol blends. Recommended for builds up to approximately 600 WHP.
• Competition Series AOS: Intended for competition-only vehicles producing higher power levels. This configuration prevents oil vapor from re-entering the intake but may require ECU calibration changes.

For street-driven or emissions-regulated vehicles, the Street Series AOS is the appropriate choice.

Key Features

• Direct-fit air / oil separator for supported Subaru turbo models
• Reduces oil vapor entering the intake system
• Improves crankcase ventilation under boost
• Continuous oil return design for minimal maintenance
• Suitable for street-driven and performance applications
• Durable construction with vehicle-specific fitment

Vehicle Fitment Overview

Designed for turbocharged Subaru platforms including WRX, STI, Forester XT, Legacy GT, and Outback XT models. Full year-by-year compatibility is listed in the Vehicle Fitment tab above.

Please note that the IAG AOS is not compatible with most aftermarket strut tower bars.

What’s Included

• IAG Air / Oil Separator assembly
• Vehicle-specific mounting bracket
• Pre-formed oil and crankcase hoses
• Required fittings and installation hardware
• Installation instructions

Frequently Asked Questions

Why do I need an air / oil separator?

Oil vapor entering the intake system can lower effective fuel octane and increase the risk of engine knock, especially under boost. Over time, oil vapor also contributes to carbon buildup in the intercooler, intake manifold, and intake valves. An AOS helps separate oil from crankcase gases before they are recirculated.

How is an AOS different from a catch can?

Traditional catch cans collect oil vapor but require periodic manual draining. The IAG AOS uses a continuous oil return design that routes separated oil back into the engine, eliminating the need for routine draining while maintaining proper oil levels.

Which AOS is right for my vehicle?

Street-driven vehicles and builds under approximately 600 WHP should use the Street Series AOS. Competition-only vehicles producing higher power levels may benefit from the Competition Series AOS, which may require tuning changes.

Does this AOS require tuning?

The Street Series AOS does not require tuning changes. Competition Series AOS configurations may require ECU calibration adjustments depending on the setup.

Will this fit with a strut tower bar?

The IAG AOS is not compatible with most aftermarket strut tower bars. Clearance should be verified prior to installation.

Installation Notes

• Professional installation is recommended
• Proper hose routing is critical for correct operation
• Always verify vehicle configuration and clearance before installation

Recommended for Engine Builds

This IAG Air / Oil Separator is strongly recommended for Subaru EJ-based engine builds running increased boost, forged internals, or closed-deck configurations. Proper crankcase ventilation and oil vapor control are critical for maintaining engine reliability in high-performance applications.

• High-performance EJ25 short block builds
• IAG EJ25 long block engine packages

Learn more about IAG Performance engine and reliability solutions