yes, in "5" the issue with fuel recommendations was largely just with the Fast and Very Fast settings turned On.
However, their recommendation based on Normal wear can sometimes be wrong, because we don't drive the same lap after lap. GT6 seems a lot more responsive to intentionally trying to save fuel, for instance.
We used Fast on Saturday and I found that the recommendations were correct each time, though I grabbed a little extra on my last stint even though I didn't need it, just to be sure.
So, rule of thumb should always be "grab a few more liters than it tells you", whether it is Normal, Fast, or Very Fast tire/fuel.
IMO the disparity between Normal, Fast, and Very Fast is fixed in GT6. If it makes a mistake in recommendation, it will be similar across all three types, and caused by a change in driving behavior, losing a drafting partner etc...
Good luck