Hello,
This is going to be a long post due to the backstory involved here. The short of it is that I'm trying to find a gear oil that will work in the 5MT in my 2005 Subaru Impreza 2.5RS. I'm going to provide a little knowledge about the tranny, since it's kind of odd, and talk a little about what I've already tried.
First of all, the Subaru 5MT is an odd little beast. The manual calls for a 75w90 GL-5 oil. The same oil bath lubes the front differential (with hypoid gear), the center differential, and the gearbox/synchros. The 5MT is a failure prone gearbox and the Subaru community regards it as VERY picky about the oil you feed it. Most people have little to no success with synthetic gear oils such as Motul, Mobil1, Royal Purple, etc. The commonly held belief is that this is because the synthetic oils are too slippery for the synchros to function though, as I'll explain below, I have reason to doubt this.
I personally have used a traditional dino 75w90 gear oil, the Redline 75w90NS gear oil, and a crazy mix called Uncle Scotty's Cocktail (consists of 1qt Penzoil Synchromesh, 1qt Redline Lightweight Shockproof, and 1.7qts Castrol 80w90 Hypoy C gear oil). I haven't been happy with any of these in the long run. The OEM dino oil fill was very thick on cold mornings and too considerable force to shift the gear lever. The Redline 75w90NS was glorious at first, performing wonderfully, until at about 5,000mi it started to become hard to push the switch gear into the new gear. It was like the synchros weren't causing the box to equalize and therefore weren't allowing it into the new gear. The Cocktail has allowed for consistent shifting, but it feels like #@$%!, it a bear on cold mornings, and the box just feels overall "sticky".
Looking for something better, I contacted Redline. Dave at Redline recommended that I try the 75w90NS. I replied with my personal experiences using it and brought up the Cocktail I now have in the tranny. He found this very interesting because his calculations showed that the Cocktail actually had less friction than the 75w90NS. This goes against the conventional wisdom in the Subaru community that the 5MT dislikes synth oil due to slipperiness. He then told me that at the operating temperature of that transmission the Cocktail was significantly thicker than the 75w90NS. I began to wonder then if the problem with the synthetics many people had been using was that they were too free-flowing. This would explain why most dino oils would work (they'd be thicker at the relatively low temps in the 5MT) as well as why people who are running Redline Lightweight or Heavyweight Shockproof are having good results (again, thicker at operating temps).
What do you guys think of the theory so far?
Since I absolutely loved the 75w90NS with the exception of the eventual hard shifts, I was wondering if using a majority of 75w90NS and then adding a bit of 75w140NS to it to thicken it up a bit would be a worthwhile experiment? What about 75w90NS plus either Light- or Heavy- weight Shockproof?
Just looking to bounce some ideas off people who know more than I do!
Thanks guys,
Ty Williams
This is going to be a long post due to the backstory involved here. The short of it is that I'm trying to find a gear oil that will work in the 5MT in my 2005 Subaru Impreza 2.5RS. I'm going to provide a little knowledge about the tranny, since it's kind of odd, and talk a little about what I've already tried.
First of all, the Subaru 5MT is an odd little beast. The manual calls for a 75w90 GL-5 oil. The same oil bath lubes the front differential (with hypoid gear), the center differential, and the gearbox/synchros. The 5MT is a failure prone gearbox and the Subaru community regards it as VERY picky about the oil you feed it. Most people have little to no success with synthetic gear oils such as Motul, Mobil1, Royal Purple, etc. The commonly held belief is that this is because the synthetic oils are too slippery for the synchros to function though, as I'll explain below, I have reason to doubt this.
I personally have used a traditional dino 75w90 gear oil, the Redline 75w90NS gear oil, and a crazy mix called Uncle Scotty's Cocktail (consists of 1qt Penzoil Synchromesh, 1qt Redline Lightweight Shockproof, and 1.7qts Castrol 80w90 Hypoy C gear oil). I haven't been happy with any of these in the long run. The OEM dino oil fill was very thick on cold mornings and too considerable force to shift the gear lever. The Redline 75w90NS was glorious at first, performing wonderfully, until at about 5,000mi it started to become hard to push the switch gear into the new gear. It was like the synchros weren't causing the box to equalize and therefore weren't allowing it into the new gear. The Cocktail has allowed for consistent shifting, but it feels like #@$%!, it a bear on cold mornings, and the box just feels overall "sticky".
Looking for something better, I contacted Redline. Dave at Redline recommended that I try the 75w90NS. I replied with my personal experiences using it and brought up the Cocktail I now have in the tranny. He found this very interesting because his calculations showed that the Cocktail actually had less friction than the 75w90NS. This goes against the conventional wisdom in the Subaru community that the 5MT dislikes synth oil due to slipperiness. He then told me that at the operating temperature of that transmission the Cocktail was significantly thicker than the 75w90NS. I began to wonder then if the problem with the synthetics many people had been using was that they were too free-flowing. This would explain why most dino oils would work (they'd be thicker at the relatively low temps in the 5MT) as well as why people who are running Redline Lightweight or Heavyweight Shockproof are having good results (again, thicker at operating temps).
What do you guys think of the theory so far?
Since I absolutely loved the 75w90NS with the exception of the eventual hard shifts, I was wondering if using a majority of 75w90NS and then adding a bit of 75w140NS to it to thicken it up a bit would be a worthwhile experiment? What about 75w90NS plus either Light- or Heavy- weight Shockproof?
Just looking to bounce some ideas off people who know more than I do!
Thanks guys,
Ty Williams
