FINE I LOOKED IT UP. Because neither of you were doing it.
Dumblederp is right, there is no research that indicates a lack of oil could cause greying.
Greying is only caused by age, genetics and oxidative stress on DNA caused by free radicals (not a lack of oil).
I responded at the tail of this back and forth, but in case no one cares to click through that conversation - you are incorrect.
No research indicates a lack of beard oil could cause greying.