You do not have the huge temperature differentials that cars have. Most of
the time you are in the 40-75 degree ambient temperature range.
Bill

"Dan Olstad" wrote in message
ink.net...
As the season draws to a close I am thinking about fluid changes in my new
to me boat. I have 1987 Mercruiser 260s and the manual recommends
straight
30W oil. Why not a 20-40W or a 20-50W? The manual says that's ok if
straight weight is not available but it is not recommended. Can anyone
shed
any light on this for me? Thanks.-DanO