I'm not sure I see any reason for a straight-90 unless you are pushing some insane horsepower and temperatures through there, but I will always defer to the OEM/differential manufacturer when in doubt.
Per the above recommendations, there are a lot of 75w140s to choose from:
Castrol Syntrax Limited Slip
Gulf Competition LS
Millers CRX LS NT
Millers CRX NT
Motul Gear Competition LS
Gulf Syngear
Fuchs Titan Sintopoid LS
Renewable Lube
Red Line
Amsoil Severe Gear
Motul Hypo Synt LS
Motul Nismo Competition Oil 2189E
Liqui-Moly Vollsynthetisches Hypoid-Getriebeöl
Royal Purple Max Gear
Torco SGO
Willall WR35DF
Mobil 1 Synthetic Gear Lubricant LS
FWIW, my M3 specs 75w140 in the rear diff (which has a GKN Visco-Lok LSD). Factory fill is the Castrol I listed up top.
Have you ever considered something from Renewable Lube? Their 75w140 is very affordable and is using very stout basestocks.
Per the above recommendations, there are a lot of 75w140s to choose from:
Castrol Syntrax Limited Slip
Gulf Competition LS
Millers CRX LS NT
Millers CRX NT
Motul Gear Competition LS
Gulf Syngear
Fuchs Titan Sintopoid LS
Renewable Lube
Red Line
Amsoil Severe Gear
Motul Hypo Synt LS
Motul Nismo Competition Oil 2189E
Liqui-Moly Vollsynthetisches Hypoid-Getriebeöl
Royal Purple Max Gear
Torco SGO
Willall WR35DF
Mobil 1 Synthetic Gear Lubricant LS
FWIW, my M3 specs 75w140 in the rear diff (which has a GKN Visco-Lok LSD). Factory fill is the Castrol I listed up top.
Have you ever considered something from Renewable Lube? Their 75w140 is very affordable and is using very stout basestocks.