Lawyers for Oksana Grigorieva want to question Mel Gibson's controversial dad and his soon-to-be ex-wife Robyn under oath, RadarOnline.com has exclusively learned.

The pair is set to be served with subpoenas to appear before a deposition in Mel's child custody war with his baby mamma.

Hutton Gibson -- an ultra-conservative Catholic who has made a series of racist rants and once declared Pope Benedict XVI is gay -- will be questioned about what Mel confided to his father about the Russian musician and what led to the pair's now infamous blow-up.