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I have an aging (but wonderful) 2004 Subaru Impreza WRX with a manual gearbox. For some time, I have noticed that the synchros have begun to feel worn with the stock dealer-issue oil. Shifts are slower to the point of being chunky going into or out of fourth gear.

I have heard about Redline Lightweight Shockproof and was looking for two answers:

  1. Is this an appropriate oil for the transmission in question (i.e., any risks)?

  2. Is this an appropriate oil for the symptoms?

Basically, this is my daily driver with an enthusiastic (but not idiotic) driver. I'm looking to improve the cars quality of life without any intention of dropped clutches at a green light.

EDIT to follow-up on the answer: I've tried to keep this car up nicely. All scheduled maintenance is up to date with 110000 miles (and now 190K miles!) on the odometer. Things like oil, suspension and exhaust work I handle myself but I let the techs have some time to play as well....

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Lightweight Schockproof is a frequent choice of many Subaru owners on both the NASIOC and SubaruForester fora. It is an appropriate choice for all Subarus.

You don't mention if you've kept up with regular maintenance, specifically whether the transmission oil has ever been changed. If it has not been changed, new oil will have a very noticeable affect on shifting and certainly will smooth the notchy feeling. If it has been changed it may be a good time to investigate further for damage to your transmission.

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Subaru transmissions are picky. You'd be safest going with Subaru's Extra S, but it can be hard to get. Not all dealerships carry it.

Redline Lightweight Shockproof and Motul Gear 300 are the only other oils I'm aware of Subaru owners having mostly good experiences with.

Whatever you do, don't try anybody's "cocktail". (if you visit the Subaru forums enough you're bound to bump into a few transmission oil cocktail recipes)

I own a 2001 Impreza RS that had the 4 speed automatic tranny. A couple years ago I swapped it for a 5MT from a early 90's Legacy with over 250k miles on it. That tranny is pretty noisy, probably a failing bearing, possibly on the input shaft.

I put Redline Shockproof in it for the first fill, and it performed very well. I didn't have any trouble in the winter, and the noise of the transmission was reduced. Now I have Extra S in it, and the noise is slightly louder and has a different tone. I have not gone through a winter with the Extra S, however, so I'm still not certain what the next fluid change will be.

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    seriously on the cocktail - that's what motivated the original question: the crazy talk you'd see in those forums, especially since a lot of the main talkers are on my ignore lists....
    – Bob Cross ♦
    Commented Jun 7, 2011 at 23:58
  • @Bob: ya, and it was more than conclusively shown that the cocktails were not a good thing to run in people's transmissions. I also added my personal experience to the answer here.
    – quentin-starin
    Commented Jun 8, 2011 at 6:40
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I haven't used this specific lube, but in my old '86 Subaru GL-10 I did put in the lube that Red Line recommended for it, and I was extremely happy with the results. I had also had some shifting difficulties where the synchros weren't working, and the Red Line MTL solved those problems. Plus, in the very cold weather, I had no shifting problems that I'd seen previously.

In my experience the Red Line transmission lube lives up to it's hype.

As far as whether it's appropriate for that car, ask the folks at Red Line HQ. They were very helpful with recommending the right weight and type of lube for my GL-10.

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