Are the guys that worry about leaving some old
oil in the cooler the same ones that start their engines and then immediately rev it to 4,000 RPMs? I'd be more worried about the air left in the lines. Look at it this way, the crankcases are big and the coolers are small. If you're really worried about leaving in 5 to 10% of the
oil change it 5 to 10% more often.
Bob