The old vacuum cleaner in the fill port is something any dealer should know. Very slight loss even if there is a stuck gasket. However what models. I thought the ones that require changing a filter that dumps out a lot of oil also require oil change at the same time. Like me - first time changing filters on my L5740 I removed the hydraulic filter and got almost zero oil. Then I thought I read here you also change the hydrostatic filter and there would be no problem. Major oil spill so I tightened the old filter again and changed with very little additional loss. Then I took out the book. Hydraulic oil filter change at 50 hours, oil change and filter change at 400. CAUTION: Do not remove the hydro filter without first draining the oil or you will regret your foolishness. Lesson learned over and over - read the book.