I'm on my second season after rescuing a 1989 Bayliner 1750 with the Ford 2.3 engine. The first season I did 30 hrs on 10W30 and have changed to Shell Rotella T4 15W40 and see if there's any change in the oil analysis. The manual recommends SAE 30, but oils have come a long way since this engine was made. The engine basically gets warmed up for 5 min, then I run it at 4200 RPM for the most of the time. What do you guys think Shell Rotella T4 15W40 or Shell Rotella T1 SAE 30?