240 differential (final drive) gear oil question

[myspv]

In my search I have gotten conflicting answers to my question about what gear lube to use in the differential. (This car does not have limited slip, so I'm not worried about that)
According to my Bentley manual Volvo specifies SAE 90 weight. Around here, the closest thing I have been able to find is SAE 80w90. Bentley says, "Volvo recommends using oil with a viscosity rating of SAE 80w when temperature remains consistently below 15 degrees F (-10C)." In Philadelphia that is definitely not the case.
So, where should I go for this SAE 90? Dealership? What are you guys using?
Bentley lists a Volvo P/N 1 161 329-6. I haven't called yet to see if they carry it.

[billofdurham]

You can safely use 80w90.

Red Line do a 75w90 that exceeds the spec used by Volvo.

http://www.redlineoil.com/products_gearlubricants.asp

Bill.