Slag formation occurs due to accumulation of inorganic materials such as vanadium pent oxide, alumina or silica. This is due to presence of vanadium, aluminum or silicon in the fuel oil. Presence of sulfur does not contribute to slag formation from my experience. However, it does contribute to corrosion and pollution. My suggestion for you is to have the slag analyzed for its constituents. I don’t have any published references on this subject.