I bought a lightly used 15 year old Monaco Coach with a Rockwell 20,000# rear axle. The 15 year old manual calls for 140w axle lube but says 90w140 is OK. I bought a 5 gallon bucket of Mobil 85w140 today thinking that 85-140 was the preferred viscosity. I live in Arizona and plan to pull a 7000# boat trailer. What do you think? Should I exchange what I have for the 140 or have oils improved enough in 15 years that what I have is good enough and may even improve my mileage?

At temp, it is going to be 140.

TRouble is, how do you get to temp?

Per my ExxonMobil manual for gear oils
90 is 13.5 to <18.5 cSt at 100C
85 is 7 to 11

140 is 24 to <32.5

I'd get the right stuff.

The Mobil would protect it fine, don't worry, it would have far better flow characteristics and far superior shear strength compared to the straight weight 140. I have a 69 Nissan Patrol and use 85W-140 GL5 with excellent results.

There is a reason Rockwell spec'd a 140 or 90W-140. I'd go with what the part manufacturer recommends. I'm sure ExxonMobil makes a full line of automotive gear oils that includes a 90W-140. I'd return it for the right oil.

The 85W- will have less resistance to flow (aka "viscosity"), but is easier flow always better? Is there an engineering reason Rockwell says don't go lower than 90W-?

I'd get assurance from Rockwell, in writing, before I made that change. Or I'd just use the recommended oil.