Also if you can get at it, take an IR temp gun reading of the temp of your oil filter after running on plane. If you have high oil temps then after break in there may be some advantage of using syn due to the superior high temp characteristics of it vs conventional. Which brings up a question, what is normal oil temp on these engines? I'm thinking it should be at least 180-200 to burn off contaminates and fuel dilution but not more than 250* I'd guess....thoughts?
BTW before switching to the Merc conventional 25/40 I used both straight 30 and 40 with no problems in starting as cold as 45 * F. So the straight weights are still a viable alternative, given the narrow air temp range for marine engines, unless you are fishing in the winter. Personally I'd like to see a direct comparison between the Merc 25/40 and the Rotella/Delo/Pennzoil 15w/40 to see if there is really any benefit to the slightly more expensive Merc stuff.
and as far as I know, the only ones that do not use friction modifiers, are the straight weights and the Merc 25/40.