Hi, i had the same problem with my islander too. Might sound crazy but when i bought the boat , the motor had a fram oil filter on it, It would idle at 60 psi, A buddy of mine at work is a boat mechanic, has been for over 20 years, We changed the oil and filter, put 25/40 in it, we put a mercruiser oil filter on, It changed instantly, 35 idle- 65 wide open. He said the reason why was because the fram filter constricts the oil flow to much, He told me not to ever run an automotive filter again! He said mercruiser filters were made not to restrict the flow like auto filters, for the purpose of trolling and keeps the engine cooler. ALSO he said the people that run the auto filters beware, The boats that run them tend to start running alittle hotter over time, and that causes oil slugde build up in the oil pan, you know whats next, plugged oil pump, blown motor.