Being as your boat is a 90, I would say no unless you changed out seals. I know this applies to car engines, however, I cannot say for sure with a boat engine. If I were you, I would call a dealer and ask to be sure. Maybe one of the dealers here can chime in. Most older engine's seals will not stand up to the straight synthetic oil. They will at first, but the seals will also begin to lose their seal and leak eventually. Personally I would just stick with the standard synthetic blends that are sold. Also, unless you've used synthetic from very start (which wasnt an option in 90), there isn't much benefit in changing over except for a longer oil life.
(Message edited by bac on April 04, 2008)