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Differential Gear Oil Weight

158 Views 6 Replies 3 Participants Last post by  juswalk
Good afternoon,

I plan to service the front and rear differentials on my son's 2013 Outback 2.5 CVT. After reading quite a few posts and watching lots of videos, it sounds like people are using 75w-90 in the front and rear differentials. The shop manual calls for 75w-90 in the rear and 75w-85 in the front. I plan to follow the service manual but wanted to see what the reason for using 75w-90 in the front ands rear is. Is it just for simplicty?
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2012 OB , 2017 Impreza
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In general:
  • Using synthetic over dinosaur oil will improve MPG due to less 'churn drag' within the gearbox
  • Using lower viscosity over higher viscosity will improve MPG due to less 'churn drag' within the gearbox
  • I would use the SAME fluid in both front/rear differential for simplicity. (AUTOMATIC XMISSION)
  • I would ABSOLUTELY use Subaru fluid in the front with MANUAL XMISSION
    • In previous postings I have described, in detail, how Subaru manual xMission SHARES fluid with front differential and why the fluid must meet critical criteria in order to lubricate both properly.
 

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Discussion Starter · #3 ·
In general:
  • Using synthetic over dinosaur oil will improve MPG due to less 'churn drag' within the gearbox
  • Using lower viscosity over higher viscosity will improve MPG due to less 'churn drag' within the gearbox
  • I would use the SAME fluid in both front/rear differential for simplicity. (AUTOMATIC XMISSION)
  • I would ABSOLUTELY use Subaru fluid in the front with MANUAL XMISSION
    • In previous postings I have described, in detail, how Subaru manual xMission SHARES fluid with front differential and why the fluid must meet critical criteria in order to lubricate both properly.
Thanks
 

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Sometimes the slight difference is the gear oils is due to different additive packages and grades (Like GL-1, 2, 3...). I would make sure that whatever gear oil you use also matched the "other" requirements than just the oil weight. This may be more important in older cars that have metals that are not compatible with some gear oil additives.
 

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Discussion Starter · #5 ·
Sometimes the slight difference is the gear oils is due to different additive packages and grades (Like GL-1, 2, 3...). I would make sure that whatever gear oil you use also matched the "other" requirements than just the oil weight. This may be more important in older cars that have metals that are not compatible with some gear oil additives.
Thanks for the info. In the shop manual the front a rear difs have "GL-5" next to the oil weight. I assumed it was Subaru speak for that specific oil but both weights have GL-5 written next to it. What does GL-5 tell me? thanks
 

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What does GL-5 tell me?
"GL4" -vs- "GL5" gear oil is SAE designation (Society of Automotive Engineers)
NOTE: The SAE has proclaimed standards for engine-oil, grease, fasteners and many other aspects of mechanical devices.

Essentially, these two designations refer to additive-package which makes the lubricant suitable for specific applications. (Not unlike my reference above regarding TRANSMISSION -vs- DIFFERENTIAL applications)

Be aware that some lubricants may meet BOTH GL4 and GL5 specifications.

Here are a couple weblinks to help explain:
 

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Discussion Starter · #7 ·
"GL4" -vs- "GL5" gear oil is SAE designation (Society of Automotive Engineers)
NOTE: The SAE has proclaimed standards for engine-oil, grease, fasteners and many other aspects of mechanical devices.

Essentially, these two designations refer to additive-package which makes the lubricant suitable for specific applications. (Not unlike my reference above regarding TRANSMISSION -vs- DIFFERENTIAL applications)

Be aware that some lubricants may meet BOTH GL4 and GL5 specifications.

Here are a couple weblinks to help explain:
That's really helpful. thanks!
 
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