Radium Engineering Air Oil Separator - Return Kit | 2015-2021 Subaru WRX STI (20-0607)

EJ turbo Subarus are famous for two failure modes, and oil in the intake feeds directly into the scarier one.

Blow-by vapor coating the intercooler and intake charge lowers effective octane exactly where an EJ's ringland tolerance is thinnest. The Radium Engineering AOS-R Kit for Subaru is the air oil separator answer for the 2008-2014 WRX and STI, 2009-2013 Forester XT, and 2015-2021 STI: a fully serviceable AOS that condenses oil vapor out of the crankcase ventilation circuits and returns the collected oil to the engine automatically, with all parts needed to complete the installation in the kit. No dipstick schedule, no draining, just continuous separation and return for the life of the engine.

What You'll Feel

• Intake, intercooler, and throttle body protected from the oil film that erodes EJ knock margin
• Zero-maintenance operation, with separated oil returning to the engine instead of collecting in a can
• Cleaner charge air effectively raising octane on a knock-sensitive platform
• A fully serviceable unit that opens for cleaning when the years add up
• Coverage across the EJ generations, from 08 WRX hatches to the final 2021 STI

Features

• Air oil separator with automatic oil return to the engine (AOS-R architecture)
• Fully serviceable design
• Includes all parts needed to complete the installation
• Fitment: 2008-2014 WRX, 2008-2014 STI, 2009-2013 Forester XT, 2015-2021 STI
• 2015+ WRX (non-STI, FA20) and 2015+ FXT use Radium's separate FA-platform AOS-R kit

The Details

The AOS-versus-catch-can decision is philosophical, and Radium builds both: a catch can holds what it separates and asks for periodic draining, while the AOS-R condenses vapor and returns the oil to the sump continuously, suiting owners who want set-and-forget protection and engines whose blow-by volume would fill a can quickly. On the EJ platform specifically, the payoff is knock margin, since oil-contaminated charge air lowers effective octane on an engine whose ringlands do not forgive detonation. Fitment splits by engine family, with this 20-0607 serving the EJ cars listed and the separate FA-platform kit covering 2015+ WRX and Forester XT, so match the kit to the engine, not the badge year. As with any AOS in cold climates, insulating considerations apply where sustained freezing temperatures are routine.

Kit Includes

• 1x Radium AOS-R air oil separator
• All mounting hardware, hoses, and fittings needed to complete the installation

Applications

• 2008-2014 Subaru Impreza WRX and WRX STI
• 2009-2013 Subaru Forester XT
• 2015-2021 Subaru WRX STI
• 2015+ WRX (FA20) and 2015+ FXT use the separate FA-platform kit