Interpolate from 60 to 90 is not going to work smoothly without
tools like reclock. For best results you need fps footage which multiple results in the desired target fps
(sorry for my bad math-english)
24fps reclocked to 30 fps multiple 3 to achieve 90 fps,
otherwise audio gets jagged (pitchy, not sync) and/or microstutters appear more or less obvious.
So theoretically (I'm no pro on that subejct though)
you would need to cut the 60 in half first to achieve this.
But then the loss is probably more then gain...
Best solution for cases like this is gsync or freesync but i don't know whether this is working on HMDs. I guess this is exactly why plp on the forum ask for an adjustable refreshrate feature for the 8k.