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Originally Posted by T1M
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I think D4 and D6 contain low levels of friction modifiers as marketed by Redline when they say this fluid has "superior synchronizer performance." The low friction synchros in an N55/1M 45bz box specifically states not to use friction modified fluid in the gearbox.
Shifting was worse for me in my N55 when switching to a D6/MTl mix and 2nd gear synchro started grinding on me at 100k. Seemed like there was too much slip and gears weren't aligning fast enough for quick shifts.
I put straight D4 in my replacement N55 box and it doesn't seem much better. Stock fluid felt best. I think I am going to drain and fill with FFL-3 for piece of mind since this is the OE equivalent fluid that presumably contains a different add-pak then Redline's ATF oriented formulas. The only thing D6 and FFl-3 have in common is the the viscosity is relatively close in the cSt 6.x range. That doesn't say ANYTHING as to what kind of friction modifiers may or may not be added to each product...
I will point out that even if the formula of the Redline DCTF fluid is better suited for use in an N55/1M box, the viscosity is actually much higher then FFL-3. It has a viscosity rating of 8.x cSt. That lands it between the viscosity range of their D4 ATF and MTL fluid and is 2 points higher than OE BMW and FFL-3. I'd expect worse cold start performance out of the DCTF fluid. This is important to me as I daily drive in the New England area.