Some years back, I took and evening class in small engine repair; in the discussion on oils, the instructor referred to a finding in some study that there were rare circumstances where the residual oil remaining in the engine could react adversly with the new oil producing what the article referred to as a "coffee ground effect", i. e. hard coffee gound like particles being formed.
No idea what produced the result--whether it was conflicting additives or what, it was just a result observed in rare circumstances. After that, I have been cautious about changing brands.
Vaughn