The 2008 STI uses 75W-90 GL-4 gear oil in its 6-speed manual transmission.
In this article, we explain why GL-4 is specified, how to select a suitable fluid, and practical tips for checking and changing the fluid, including notes about the differential and maintenance timing.
Recommended transmission fluid
Key considerations when choosing the right fluid include the grade, the GL specification, and compatibility with the stock limited-slip drive. Here are the core requirements you should follow for the 2008 STI's 6-speed manual:
- Fluid type and spec: 75W-90 gear oil that meets GL-4 specification. Avoid oils marketed only as GL-5 unless the manufacturer explicitly confirms GL-4 compatibility.
- LSD compatibility: If your car has the stock limited-slip differential, use a GL-4 gear oil that is compatible with the LSD without requiring separate friction modifiers, per Subaru's guidance for this model year.
- Quality and viscosity: Synthetic 75W-90 GL-4 oils offer better cold-weather performance and longer service life than conventional mineral oils.
- Availability and labeling: Choose a product that clearly states "75W-90 GL-4" on the label.
- Maintenance context: Use fresh fluid when changing and avoid contamination during the process.
In short, select a 75W-90 GL-4 gear oil from a reputable brand, ensuring GL-4 compatibility and LSD suitability for your build.
Notes on your STI build
Some 2008 STI models include features like a limited-slip differential or driver-controlled center differential (DCCD). For these configurations, stick to GL-4 oils that explicitly support LSD and, if in doubt, consult the owner's manual or a Subaru service bulletin for the exact specification for your VIN.
How to change or check the fluid
Following proper procedure ensures a thorough and clean swap. If you're not comfortable performing this maintenance, consult a qualified technician.
- Warm the car and safely lift it to access the transmission drain and fill plugs.
- Place a drain pan underneath, remove the drain plug, and allow the old fluid to drain completely.
- Inspect the drain plug and magnet for metal debris; reinstall the drain plug with a new washer if recommended by the service manual.
- Remove the fill plug and add new 75W-90 GL-4 gear oil until the level is correct at the fill hole.
- Reinstall the fill plug, lower the car, and test the transmission by shifting through the gears with the engine running and the parking brake engaged.
Always use clean, uncontaminated oil and a proper funnel or pump to minimize air entrainment. If you have LSD or a DCCD-equipped model, verify you’re using the correct fluid specification for your configuration.
Maintenance considerations and tips
Regular checks and timely changes help preserve shift quality and drivetrain health. Use Subaru’s schedule as a baseline, and adjust for aggressive driving, harsh climates, or towing if applicable. When in doubt, follow the service interval recommended in your owner's manual or consult a Subaru-certified technician.
Summary
For a 2008 Subaru STI, the recommended transmission fluid is 75W-90 GL-4 gear oil. Do not use GL-5-only formulations unless explicitly approved for GL-4 compatibility, and ensure the oil is suitable for your model’s LSD/DCCD configuration. Follow proper drain-and-fill procedures, and refer to the owner's manual for exact instructions and maintenance intervals.