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.
I spoke to their technical staff on the phone, and was specifically told that FCA doesn't release the spec. Now obviously this is one of those "proceed at your own risk things" since I'll only tell you what I've done and what worked for me and others, not go against AMSOIL's recommendations, but when I spoke to them about running 75W-90 Severe Gear in my 124 Spider, they basically indicated that the friction modifier package was the only thing I'd have to worry about.Based on the Amsoil reply I received, it seems they know the spec but don't have an oil that meets it:
"Subject: RE: Technical Service Contact Form CRM:0234911
We do not list a product because we do not have a product that meets the spec they are looking for.
AMSOIL Technical Services
Tech Line: 715-399-8324
Email: [email protected]"
That said, has anybody tested a virgin sample of the MOPAR/Fiat oil (Blackstone, etc.) to see what it is?
And thanks for your feedback on this thread, I appreciate healthy discussion to hash things out where we can all learn. I'm not sure I would discount a tradesman that looks at diffs daily and can tell performance by looking at wear patterns and catastrophic failures and know what oil they were using to draw some conclusions (where theory and reality meet), but on most of your points I agree, I have used Amsoil for the past 20 years.