So long as we are revisiting this old thread, I would like to add that the “blending tolerances” for motor oil, even within the same major brand, batch to batch, can be shockingly wide.
I happened to notice a dramatic difference between two bottles of Castrol GTX of the same rating as I was filling my car. One looked like 20W at room temperature and the other looked like 90W at room temperature. Just sloshing the oil around i the bottle, they were unmistakably, dramatically different. I sent the two bottles back to Castrol (actually to the lab of the owner company BP), each with about a half inch of oil left in it. After about eight weeks, they responded, advising me that both containers were “within blending tolerances”. Of course, that was what I expected them to say. They were not going to volunteer that they had a quality control problem and open themselves up to being blamed for engine failures.