I have the yanmar 3gmD. I think the D stands for direct sea water cooling where a 3gmF stands for fresh water cooling. I was also confused by the different oil spec in the manuals. On my transmission I changed oil using a 40 WT as this is what a Yanmar dealer told me to use. Since then I did find the name plate on the Kanzaki (V drive transmission) that says use 15-30 wt. I have not changed the trans oil since seeing this new information and have run 50 hours without problems- maybe I should change to the 15-30 wt. I agree the oil specs are confusing- maybe I should contact Yanmar factory and see what they say.