Adding Cetane booster or additives

This is a really dumb question. Should additives or Cetane boosters only be added to the tank when filling it with diesel, or is there sufficient diffusion to allow one to add the appropriate amounts at other times?

Thanks

it would be ideal to add the mix just before you fill up.

Its probably OK to add at other times. The mixing may be less complete though.

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96 TDI Wagon, ABS, emerald green
00 Jetta TDI GLS, silver arrow, CWP, LUX1, etc, etc.