Benifits of Synthetic Oils in Diesels

I have a marine diesel application with 2 different engines on a sailboat.
One is a Lehman (marinized Peugeot) and the other is a Fisher Panda Genset
(Farymann W18).  I have used Shell Rotella 15W40 in both with ok results -
not sure what bad results are.  Any good reasons to move to syn oils?

With marine applications the engines are typically run at much higher
loads
perhaps 70% to 80% of max horsepower for hours (or days).  Cars a much
less
stressed.  Also, there always seems to be the salt water corrosion
problems.
What do you all think?

AMSOIL makes a 15W40 syn diesel oil.  Can this be mixed the small amount
of
Shell after an oil change?