Tough to measure exactly, because variables have to be the same except for the gearbox oil used.
Wind, tire pressure, temperature, weight in car, are the major things that affect efficiency.
One thing I might try is to drive to a suitable hill, without interfering traffic near me, and do a "Coast Down Measurement" in Neutral, to see how far it coasts. Then change the gearbox oil and try it again to see the difference. Then, we'd know what energy efficiency gains we have.
About those Coast Down tests, I do know a fellow at NREL (a place that does EV research) who knows somebody at INL (Idaho federal lab like NREL) that does more of those coast down tests than NREL does. I'll ask them if they know what gains can be had with thinner gear oil. ( In nerdy engineer-speak, I'll ask them what the partial derivative of Range is with respect to gearbox oil KV100 in the Leafs they've tested before. )
https://www.energy.gov/sites/prod/files ... 2015_o.pdf
They have tested all kinds of EVs in Idaho in the last 12 years. Your taxpayer dollars at work.
In ICE vehicles, for comparison, the use of lower-viscosity engine oil has been studied a lot for increased MPG. It's commonly known that you gain about 1.5% fuel efficiency when you use a 0w-20 (kv100 8) oil instead of a 5w-30 (kv100 11) oil. That's a "partial derivative" of delta 1.5/3 = 0.5%/kv100
If you go too thin, then you lose too much hydrodynamic oil film thickness, placing excessive burden on the EP (extreme pressure) chemical additives in the ATF oil here, resulting in more boundary (metal-to-metal) friction, increasing overall friction even though hydrodynamic friction goes down. (For a deep-dive, google a Stribeck Curve image, and see the lowest point on that curve is where the hydrodynamic drag is lowest right before the metals touch.)