ZF Lifeguard 8 fluid employs chemistry that predates 2008. They never changed it since then. Even Valvoline MaxLife is far more robust than ZF8 fluid, as many here use it successfully in their transmissions.
Valvoline tests Maxlife at 280F continuously for 60K miles in their lab every time there is a reformulation. That's the benchmark it has to pass. I have yet to see a ZF8 transmission fail on MaxLife.
I use AMSOIL ATL because I believe it to be even better than MaxLife. I reserve judgment on Red Line D6 because I haven't used it yet, however, I believe it's pretty good, though not as advanced as AMSOIL ATL.
However, one of the biggest Red Line customers are BMW enthusiasts, and they have ZF8 transmissions in their vehicles. Therefore, I believe that when it comes to the ZF8 transmissions, Red Line D6 is an excellent choice.
Just about any of the fluids I listed above is better by leaps and bounds than the Shell manufactured ZF Lifeguard 8 fluid.