Lightweight Manual Transmission & Differential Fluid
 

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.

'thinner'
 

I presume that you already have a full-synthetic lube package.
Whatever Subaru recommends, rating wise, you ought to maintain.
Years ago, I attached a half-dozen baby food jars with different lubes to a board and placed it in the freezer overnight.
Next day, I uncapped all the jars and tipped the array over a pan.
Regardless of 'cost' and rated viscosity, they all 'poured' identically, from the most expensive racing lube, to the lowliest, entry-level lube.
I'm uncertain as to whether there's any low-hanging fruit available to us, gear lube wise.

What gear oil does Subaru recommend? GL-4 or GL-5? I damaged the synchronizers using GL-5 in a two transmissions requiring GL-4. Don´t put GL-5 in a transmission requiring GL-4. GL-4 has gotten much harder to find and is very expensive. Next time I change gear oil, I will likely switch to synthetic.

I wouldn't be willing to try messing with a different specification for the gear oil in a Subaru, as their transmissions are often pointed out to not be so reliable at all.