[twj347]

It's about time to replace the gear oil in my Subaru. I'm looking to use some thinner fluids than OEM to improve economy. I live in a cool climate and don't tow anything or drive hard for extended periods, so I'm not too concerned about the fluid getting too thin. My gearbox felt pretty sluggish driving in -30 C temps this morning, and just coasting to a stop in neutral, I can tell there's a large amount excess drag in the drivetrain in cold weather. I assume this is mostly from the transmission and differentials.

Any recommendations for a thin/economical gear oil with a GL-5 rating? Most thinner oils I've found are only rated GL-4, which I'm not willing to experiment with. The thinnest GL-5 gear oil I've found is Motul Motylgear 75W80. However, despite being thin and having a decent viscosity index, its pour point is only -36 C, so I'm not sure if it's appropriate in an application where temperatures can occassionally dip into the -40's.

GM/Delco/Pennzoil Synchromesh is not GL-5 rated, but it's a popular additive for Subaru manual transmissions, so I might add a quart of it to whatever gear oil I decide to use.

Are there any studies or tests showing how much of a difference lighter gear oil can make? I would assume it's not much of a benefit once the drivetrain is at operating temperature, but I'd expect it to make a pretty big difference in cold weather and during warmup, especially with all-wheel-drive.