Graph on page 5 of this link https://www.widman.biz/uploads/Transaxle_oil.pdf shows very little difference in dynamic viscosity for 75W-90 and 75W-140 over normal temperature ranges.
Graph and table here https://wiki.anton-paar.com/en/gear-oil/ for the fully synthetic version of 75W-140.
Page 10 of the first link https://www.widman.biz/uploads/Transaxle_oil.pdf shows a comparison between synthetic and ordinary 75W-90 dynamic viscosity. Synthetic is, as is often the case a little thinner.
However dynamic viscosity is probably not the best measure of what is actually happening in a differential as what happens to an oil film under load can be quite different between differently made oils. For sure the 75W-140 will be a better oil than what was originally specified. The wider range simply means that viscosity holds up better with increasing temperature. 75W specification is the viscosity at 0°F (-18°C), the 90 and 140 numbers give viscosity at 212°F (100°C) which are both thinner than 75 at 0°F (-18°C). Far more information that you want here https://www.substech.com/dokuwiki/doku.php?id=gear_oils , click the links at your own risk! You may miss dinner.
Frankly I'd not worry. Gear oil viscosities are much less than you'd expect from experience with single weight engine oils. Found that out way back giving my Norton Commander its first home service as the book called out SAE 140 gear oil for the gearbox. I was expecting to need a spoon but it just poured!