theres a really trustworthy marine shop here in my town, and he swears by using 40wt in most the little boat motors and claims ( even after 40 years ive never had a problem with any customers ) ive done some research on oils used for the 3.0 l mercs and they are all over the place. theres people saying theyve used 530 for 10 years and others saying 50. looks like i cant go wrong. currently, i have like 3 quarts of 1540 in it and a quart of 2050. it was of what i had on the shelf honestly... ill run it this season, and maybe change it next year or so. would i be better off using a 1540 for the boat or could i use a synethic 1030 and be safe? this is an off in the future switch but id like everyones opinion. my chevy 350 in the volvo has 1540 in it.may just use it for the boats.