I was reading up on it last night and found this https://www.researchgate.net/publication/305525405_Effects_of_mineral_oil_immersion_cooling_on_IT_equipment_reliability_and_reliability_enhancements_to_data_center_operations

Hard to get a clear consensus from forums, some said mineral oil good, some bad, miners advised against in favor of $100/gal stuff 

 The expensive dielectric is definitely superior to any other option.