Back in the 80's, I took an engine overhaul class. As part of the class, everyone brought in an engine to work on. After the classroom portion of the class, the instructor asked everyone what type of oil they used, and took notes. When we began to take the engines apart, he pointed out the differences in sludge & deposits between the different oil brands - and explained why he recommended
against using certain brands. The differences were noticeable, and surprising... some engines were still bare metal inside, while others looked like all inner surfaces had been coated in brown lacquer.