Originally Posted by triger716
Google Mike Miller's Old School BMW Maintenance Schedule. He explains the difference between the two fluids quite well. I was going to copy and paste that section but since it is copyrighted I won't.
In short he says if you have a limited slip diff you need to use 75W-90. If you have a open diff you can use either 75W-90 or 75W-90NS.
He makes a technical mistake there, and there are other small mistakes in that document, I saw at least one labeling Redline motor oil base stock chemistry as PAO.
Using the NS fluid in a standard diff will increase wear, as the lack of friction modifier means... higher friction.
The regular fluid is the same as the NS fluid, but it has this friction modifier added. A certain amount is needed so the clutch plates can slip during engagement, or you get chatter. The regular fluid already has an acceptable amount for most on-road LSDs that are clutch based (note, Quaife LSDs are not clutch based)
If you get excessive LSD slip with the standard fluid, you use the NS fluid and add the friction modifier until the chattering/slip balance required has been achieved, or mix a balance of standard and NS fluid together.
In a standard diff, you want as much slipperiness additive as possible without affecting the hydrodynamic lubrication of the oil. You can add a bit more of the LSD friction modifier, or use a specific hypoid gear friction additive like the LiquiMoly 1040 to improve the non-hydrodynamic properties of the oil.
In regards to using the standard 75w90 fluid in a transmission, the friction modifiers may cause synchro problems. Once again, mixing a balance of the standard and NS fluid would be best for a synchro transmission if you want GL5 protection. Most of the manual transmissions on BMW are fine with GL4, as 75W90 GL5 is considerably thicker when both warm and cold, and will reduce power to the wheels and fuel econ..... but it will protect the gearing better. The make a shockproof gear oil for racing transmissions that is the same viscosity as a GL4 fluid but provides GL5 wear protection.