Based on 4 years and 200 hundred hours. I would use a judgment call as to what preventative maintenance Items I change. I would do the yearly oil change and reverse gear oil and filter changes. The belts personally I would inspect for cracking wear and tightness. I would look for excessive belt dust, and the condition of the pulley's. If all looks good I would go another year without replacement. I do not recall a compressor installation on the D4 inboard so no oil change. I would add to the preventative maintenance a raw water pump impeller replacement and pump housing inspection. I would also do the antifreeze replacement. The exhaust hose is another inspection component. If it looks good, shows no evidence of leaking, delimitation or deformity I would not replace it. If you do replace the hose I would go one step further while removing the hose I would also remove the elbow and inspect the turbine side of the turbo for coking, corrosion, and fouling. This area tends to get a build up and can cause the turbo to malfunction.
The manufactures guidelines should be followed. This gives you the best protection for maintaining longevity. The owner of the vessel knows best on how the boat has been used and how much it has been used. If you feel comfortable about making judgment calls about the maintenance needed based on your usage do that. If you don't then I would follow the recommendations 100%. If you are having a certified Volvo diesel technician preform the work. Let him make the call and ask questions so you have a good understanding of why the services are needed.
