How? Higher resistance. It can also hurt other parts inside, especially needle bearings..
Not sure where you get info, but here is some facts-
Early/original Jeep Quadratrac, chain runs all the time, has a "center diff." with limited slip clutches, use a tractor oil, light weight, with friction modifiers. Originally was mineral oil, I think. About the weight of 10w motor oil.
NP203, used in may trucks, many years. Full time AWD, but, also can be positive locked. Has center diff., no clutches. Uses 10-30w motor oil.
NP205, as big and heavy as a 203, but is all "gear to gear. Used in the same trucks as 203,optional. Uses 75-90 gear oil.
all NP and BW (new process/new venture, Borg Warner) that have a chain, use atf-- Except-
The NP used in newer gm's, 99 up astros, and some full size, any that have the electric motor that applies the clutch pack when slipping is measured, use Autotrak II fluid. Which is a very lightweight atf with lots of friction modifiers.
Basically, if it is after 1985, and has a aluminum case, it's a 98% chance it uses ATF.
Now, some of the bigger trucks still use a big, heavy gear to gear case, f500's and up, and such and use gear oil.
Subaru and audi, a "center diff. transfer case, with clutches that can be locked--use ATF.
The bottom line, if it has a chain, it does not use gear oil. Now there maybe an exception out there, but I have not seen it. And I've been working on these things most my life. Right now, sitting in yard, early Quadratrac, np203, np231, BW4472(all AWD astros, 90-98), and a NV136, 99-05 astros,AWD.
Woops, forgot camper truck--NP241