After talking to Intel, we finally know the truth about what RAM Intel's integrated memory controller (IMC) can support.
Skylake's IMC Supports Only DDR3L : Read more
Skylake's IMC Supports Only DDR3L : Read more
What this article fails to mention is that most kits of DDR3 rated at 1.5V can still run at 1.35V. Just at lower clocks.
So if you don't have DDR3L on hand, you are probably better off to go ahead and spring for the more expensive DDR4.