Thinnest gear oil???

What is the thinnest oil that would work at least somewhat adequate in a 8.25 open diff in a light weight very low power kind of car? Just thinking about doing some more MPG experiments and was wondering if I could use something lighter like slick-50 or maybe ATF+4 since they use it in FWD auto tranns with differential gears in them. Not nessacarily looking for 200,000 miles out of it but if I could get 50,000 - 100-000 miles out of one.

I would go to maybe a straight 30 wt. motor oil. That is what Honda used to use in their manual gear boxes.

Gus

Heck, there's lot's of RWD manual transmissions that use Dexron.

Try looking into Red Line Ultra lightweight synthetic gear oil, the green stuff

Motor oil may work, but hypoid cut gears found in the 8.25" differential require difference properties than helical or straight cut gears found in manual transmissions and transaxles. If it's relatively light duty application then a good motor oil might last.

If it's an experiment then go I wouldn't bother with slick 50. I would go straight to a 0w20.

The 80w90 gear oil has approximately the same viscosity as 30w or 40 w sae motor oil.

Take a looky.
https://www.kewengineering.co.uk/Auto_oils/oil_viscosity_explained.htm

I know a well known and accomplished drag racer that ruined a set of fairly new 4:86 gears in his 1971 Dodge Demon 8 3/4 in one run trying either 30 or50 Wt. racing oil in the rear end for one run
I use ATF in my NP833 18 spline pro shifted drag racing 4 speed for 2 1/2 years in my old 1970 Hemi Cuda, worked good, lasted a long time
I would change it about every 40 runs after doing maintenance on the gears and sliders

The cost to rebuild is not worth it. Use Red line or Royal Purple synthetic in it and call it a day.

I would not personally undertake such an experiment due to risk/reward, but.....

Start with the recommended gear oil for the 8.25 diff (90w ?)
but choose the synthetic version.

Add a package of MoS2 additive to the above.

After doing a fuel economy test of some sort.
20% dilute this original gear oil with Mobil One 50-15w engine oil.

Repeat, to 40% dilution, 60% dilution, etc.

When Chrysler changed the Ram pickup 9.25 diff to ball bearings from opposed roller bearings about 6-8 years ago they only claimed a 0.1 MPG improvement.

Maybe start with tests where you jack up the rear axle off the ground, run the rear tires up in free air to 70 mph, then time how long it takes to coast down to 35 mph?

Your test will have to be ultra-sensitive to the friction difference since it is in the range of 0.1 MPG

For the time and money you could find way bigger gains with some aerodynamic mod.

Ain’t dere some highly windy Montana mountaintop
that could stand in for a wind tunnel?

Any oil that is not GL-5 rated will cause ring and pinion gears to wear very prematurely.

Seems to me that whatever lube the ring gear hasn't thrown off is being crushed & ground by the pinion.
Might be some difference with pinion location vs. fresh lube pulled up by the ring gear, but it's a different lube requirement than an engine or trans.
For one or two hail-Mary drag-racing record attempts, you might gain .001 or so.
For any practical application, I'd bet on finding carnage.

In some cases there is always the option of running less of the proper, but lighter viscosity gear oil, to reduce friction/windage. Testing would involve figuring out what the minimum amount is. Some have told me that 1 quart is enough in an 8 3/4 application, but don't know if that's for limited runs or constant. I never tried less than 1 1/2 qts. for limited runs (drag racing). I'm sure there would be a difference between a spooled or open rear compared to a Sure Grip equipped differential.

I ran Valvoline VR1 50w motor oil on my old Roadrunner. Strip only in a Dana 60 4.88 gears for 4 years before I got rid of the car. 470 Indy heads. 6.20's 1/8 mile. Gears and bearings looked like new.