Well, I went and tested it on another game with a gardener who was near to getting life fruit, and finally got enough to put together my favorite bottle, nine life fruit and a pomegranate , and had the nectar maker drink a glass and it was good for an extra day of life, so, it's a vintage only for immortals.
Then I went and tested one life fruit in with 9 flame fruit and found that there was no life extension, just the mood boost. So, I guess the answer is to not use Life fruit in Nectar unless it goes into an "Immortals Only" room.