Of the oils that you mentioned, Castrol EDGE Syn has more(waaaay more) Magnesium compared to the others you listed. Most conventional oils only have Calcium and little if any Magnesium. Most synthetic oil have a combo of Cal/Mag. Castrol has a ton of Magnesium and less Calcium. Maybe this is why? IDK!
Or, it could be the way the DYE in the oil darkens compared to what you've been using. Again, IDK!
I don't know it the link will work however, if you look up the info in the PQIA site you will see what I am talking about. SuperTech Syn isn't listed but the ST conventional is.