Interesting topic...I was actually talking about this with my brother, Larry, and he had mentioned that some folks actually use oils directly over the primer...I would be interested in hearing a bit more about that technique if anyone would care to share. Seems to me that if one is using that technique, then the lighter primmer would be used; I dunno, just thinking out loud.
I use an acrylic undercoat to give the oils something to "bite" into not only for color, but for application, too. I try to match the undercoat (basecoat) as close to the final color as possible...so, in essence, I paint the figger twice (once with the acrylic base, then wil the oils), and suspect many of us do the same...
All the best
Patrick