I must have spent 20 minutes looking for an oil drain plug on the underside of my 4.3 liter Mercruiser engine before it occurred to me that maybe there wasn't one. I've checked the internet and while there is one, the standard approach is to pump out the oil. Hey raven! This pump worked very well. Transfer time is primarily limited by the width of the suction tube, but this is to be expected as it is narrow enough to fit over the probe tube. The only problem I had was finding the oil reservoir inside as it is a totally blind process. In the end I settled on "If the intake manifold is hot and moving like it's being tickled, you're in the reservoir drawing oil."