I'd be willing to bet the dealers who don't drain it from the plug simply suck out what they can, in other words their oil change sucks. I've done the last two myself, I drain it from the plug and it takes all of 1/2 hr. I have always put in 8 and 1/2 qts. The two times the dealer did it, both times the hi oil prompt came on after leaving the dealership, both times oil had to be drained off. The first oil change cost me around 120$, a year later the second was 160$, its a lot cheaper to DIY. Unfortunately the dealer oil techs I've dealt with just don't seem to care.