From what I've read there is no 'ideal' way to plumb your system. In addition to the items already mentioned keep these in mind:
- using individual returns to the veg/diesel tanks (i.e. 6 port valve) will cause some mixing of fuels at switchover (important if you are relying on your veg filter for wvo)
- a looped return can be setup to 'backflush' your veg filter with diesel. To do this use 2 3 port valves... you'll have a looped return when on veg & a return to tank when on diesel. If you delay switching the return/loop valve after switching the source fuel valve you'll essentially loop deisel for a short while backflushing diesel fuel into your veg filter. This has a couple of advantages... it can clear some of the filter element and it also eases the switchover to veg since your filter isn't filled with thick veg oil at switchover.

-John

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1985 MBZ 300DT
1969 MBZ 220d
1984 MBZ 300TDT
1981 VW Vanagon