Thanx for your replies.
According to this site:
http://www.boatfix.com/freshkits.asp
The rule of thump is 150 hours of sal****er use and 250 hours
freshwater use. After that it is probably too late to install a
freshwater cooling system. Is that right?
Because if is not right I may concider installing fresh water system.
The other Mercruisers here in Nuuk runs around 143 F (62 celsius
degrees). But I have read that the optimum temperature for Mercruisers
is around 76 F (80 celsius degrees). What temperature does your motors
run in in the new york area or areas?

Thanx again
Jens-Erik