I had this discussion with my engine mechanic last year because we sometimes make multi-day passages with our Grand Banks 49 (twin DD671s). He suggested checking the oil underway when the crankcase was known to be full or nearly so, and then making a note of the level on the dipstick. I've done that several times as a reference and it seems to be consistent. At least on the 671s, adding oil underway is not a problem. Regards, Wayne B GB49 CL, Long Legged Lady