I don't think it was the oil, as a synthetic, semi synthetic, or dino of the mentioned brands in this thread should all meet the specs of his engine. I'm not saying it didn't calling you a liar, however if the oil did cause a problem it was not because it was a synthetic, or semi etc, if a shop changed his oil maybe they put some contaminated oil in ? even then I don't see this causing problem that you would not see by checking the oil. I'm not sure what happened, but before I chucked it up to damage I would do some more filter changes if that did eliminate the noise for a day..the synthetic could have dislodged some sludge, or gunk/dirt...but even then with today's oils, and the ford late year model, him babying his stuff, Castrol GTX is a fine oil as well I have a hard time seeing that. Keep changing out the filter a couple times, wait a week or so do another drain/fill with Castrol to get any residual synthetic out. I'm not a fan of any oils, but I have used both oils Mobil in a 94 Ranger with a 4.0, and several other makes/models of cars, trucks, engines etc never a problem with any of them, I also ran Castrol for most of my life with no problems