This is a bit pedantic, but your comment about siphoning oil from boat engines via the dipstick tube is not strictly correct. Many boat engines, mine included, have a permanently fitted hand pump for removing engine oil. The pump is connected to the sump via a pipe to where a drain plug would otherwise be, and so does remove oil and sludge from the bottom of the sump as if through a normal drain plug. However, I agree that siphoning out oil via a dipstick tube or the like is not a preferred method.