[brucep]

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.

 

[juswalk]

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

 

[RobertD]

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.

 

[juswalk]

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

 

[brucep]

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:

• GL-4 Vs GL-5 - What Is Best Gear Oil For You?
• Gl4 Vs. Gl5: What Is The Difference? | Rx Mechanic

 

[juswalk]

"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:

• GL-4 Vs GL-5 - What Is Best Gear Oil For You?
• Gl4 Vs. Gl5: What Is The Difference? | Rx Mechanic

That's really helpful. thanks!