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Why do people like redline? It's sold to the segment. With claims of use of stouter add packs and exotic base oils, theoretically it is more stable and better performing. It hasn't exactly been my finding (other lubes are quieter in the LSD in my E30, and other MTLs perform better, longer in its gearbox. Their oils don't meet the BMW or API specs, and some recent tests even showed poor foaming performance in their engine oils.
Why a heavier/lighter lube? Well for LSDs, a BMW TSB from a while ago specced XJ as the best lube and for use in LSDs. If one desires to stray from the 140wt lubes, keep in mind that the API designation of 90wt was cut in half and the 110 designation was subsequently created. So a 90wt is a fairly light lube at this point, as the. Iscosity range for 90wt lubes was effectively cut in half. A good 75w-110 like amsoil svg would be a good compromise.
D4 vs D6? D4 was a product intended for use where dexron III fluids were specced. Dex III is no longer a valid spec, so technically there is no such thing as a Dex IIi fluid, since GM no longer licenses that. Dex VI is the replacement for ATF, and Redlines D6 product is for that. Now Dex VI is not approved by GM for MT use. I'd personally not be comfortable running the dex VI add pack in an MT without specific OEM approval. Dex VI is a much lower viscosity but much more shear stable ATF, but with different overall lube design. Dex VI is "thinner" than new dex III fluid, but dex IIi shears down rapidly to a lower viscosity. Still, I'd not be inherently trusting of the dex VI chemistry for synchros and elastomers until it is well proven. To date it is really anecdotal evidence with fairly low accrued miles.
I'd be comfortable changing diff lubes, but not necessarily MTFs.
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