Psyllium husk completely halted the flatulence for me. I take one tablespoon an hour before a Soylent meal, and furthermore do not eat solid food as part of the same meal. I see in the DIY section that some mix the psyllium into their Soylent. This would be ineffective for me, as the lead time is crucial. What happens is that the psyllium forms a clog through which Soylent cannot pass.
I saw no effect whatsoever from probiotics, drinking slowly, or skipping oil. The problem was that the liquid traversed my small intestine too quickly, and spilled into my large intestine, creating an epic smorgasboard for the flora there. I suspect this effect is more pronounced in vegetarians, who have a shorter “gut transit time” in general.
Hope this is helpful!