The red one is the low-SAPS formula designed for emissions compatibility with diesel particulate filters and 3-way gasoline catalytic converters. It's a bit more expensive than the classic-ESP.
Running a higher viscosity than needed in a rear differential will not provide any longevity benefits unless you are breaking the film, which you shouldn't be on a GL-5 gear oil. It will however generate more heat and slow down LSD engagement. Post 12 is missing some critical details; he claims hypoid gear oils haven't improved. Sure, I'd agree, if you're using conventional based cheap stuff off the Walmart shelf.
A quick google search yielded the following as an excerpt from 1998: SAE Viscosity Grades – viscosity table and viscosity chart :: Anton Paar Wiki . SAE-90 is showing as 13.13-18.5.I guess we all might read and interpret different things in that thread but what I got out of it was the same thing as another poster restating what the OP of the Miata thread mentioned:
"the old 90W had a range of 13.5-24 (average 19.5). In 2006 sae j306 changed to narrow the range of 90W to 13.5-18.5 and added 110W 18.5-24 cSt."
The OP in that old Miata thread does nothing but build differentials for a living. Amsoil lists 75w90 but also lists the 75w110 diff fluid for the old Miata NC application (2012 MAZDA MIATA (2.0L 4 -cyl Engine Code [F] L) Motor Oil, Filters and Lubricants - AMSOIL) which from the factory came with 90W......nothing is recommended for the current gen 124 or Miata on the Amsoil site so I may reach out to Amsoil technical for their thoughts. I was assuming the current gen differential was based on the NC diff since I know some have swapped ND diffs/parts into their NC but you know what they say about assumptions........we do share a variation of the NC transmission.