Effect of gear oil viscosity on transmission efficiency (Metro owners take note)

[bandit86]

I had Toyota change the manual tranny fluid when they did the ECM recall last fall. I lost 3-4 mpg ever since now you guys have me wondering... Maybe they put the wrong stuff in?

[Sean.Heihn]

Usually fuel economy goes down during winter, denser air, cold temps and lower quality (lower energy density) winter blend fuel all contribute. Toyota has been pretty consistent with 75w90 gear oil in their car manuals from what I've seen, had a 2002 Corolla that took it, Hands manual was for 95ish to 2002 Corolla, all those years took it, 2007-2011 Yaris take it. They might have put in conventional and if you had synthetic in before you might see a slight decrease in FE, maybe a few percent, but not 3-4 mpg.

[Night_Sailor]

My Volvo 240 manual transmission uses automatic transmission fluid--much thinner than gear oil.

I think you are dead nut right on the lighter oil being more effective. The only reason I can think of for thicker oil to to coat the gear better as they might spin off the lighter stuff.

If it was me, I'd go for lighter weight oil.

[euromodder]

Using thinner gearbox oil is what just about all car manufacturers do these days, to get better FE ratings.

Volvo used lighter transmission oil on its later, more fuel efficient models that use the same gearbox, so I've done the same on my V50 when it was due for a transmission lube swap.
No immediate change in FE was observed - nor expected.
This isn't the kind of improvement that will show in day to day driving.

[Madact]

Some more low-viscosity fully synthetics to add to the list (these two are manual-transmission specific oils compatible with brass & bronze synchros). Info unapologetically copied from data sheets, I'm sure the companies won't mind the free promotion

Penrite PRO GEAR 70W-75
Specifications: GL-4 Plus, ZF, TE-ML 11 (manual) MTF BOT 338 Tremec (T-56)
Typical Characteristics:
cSt 40oC 34
cSt at 100oC 6.9
Viscosity Index 171
Viscosity, Brookfield, cSt at -55oC 82,700
Zinc, mass % 0.131

CASTROL SYNTRANS FE 75W
Specifications: API GL-4, WSS-M2C-200-D2
Density @ 15C, relative ASTM D4052 g/mL 0.852
cSt 40oC 32.2
cSt 100oC ASTM 6.3
Viscosity Index 154
Viscosity, Brookfield cSt at -40C 10,000
Pour Point -51C
Closed Flash Point 226C

Redline "Shockproof Superlight" (may be incompatible with brass synchros) is claimed to "be rated as a 70W90 Gear Oil, but has the lower internal
fluid friction of an ATF".
[EDIT] There are some mixed opinions out there about shockproof, some say it may have high sulphur content, making it unsuitable for use with brass/bronze synchros. The redline site says the Redline "Shockproof Heavy" is unsuitable for synchros due to excessive slipperyness, but no firm statements either way on compatibility for the lighter grades. The "shockproof light" grade is also known as "smurf blood" (it's blue and expensive), many swear by it but others say it has bad long-term effects on synchros (sulphur issues and glazing). However, by some reports it may also temporarily resurrect a gearbox which is crunching gears. Redline MTL is specifically recommended for brass synchros, but isn't especially low viscosity (75W80).

Also, redline have an interesting article here which discusses effects on fuel economy and drivetrain efficiency:
Redline gear oil article

And another table of low viscosity transmission oils: http://www.red90.ca/rovers/MTF.htm