Will this anecdotal be of help?? We bought the GB at 13 years old and found nothing in the logs on tank inspections or cleaning. Some minor indicators from the stbrd. tank made me suspicious, so we called in a service and they pumped and polished it all and examined both cast iron tanks while I was there. Nothing whatsoever out of normal order: not sludgy, dirty or rusty. Replaced all the fasteners in the inspection ports, pumped the fuel back in, and we were done. DID NOT need to do it and all has been running fine since. We use double Racors for both engines using 2 micron filters for everyday and the reserve filter 10 mics. We change the 2 mics every 150 to 250 hours max.. (Rarely change the 10's!) Vacuum guages show zero to 2, depending "needle jiggle". Engine mounted filters on these 6BTA's are 2 or 5 mics, from what I have been told, but not sure about that. They are changed only annually. As with everyone, have been in more rough seas than one cares to remember (offshore, Alaska, etc..); no rough running engines or shutdowns - ever.
We also have quarter turn ball valves at the bottom sumps of both tanks and connecting to the delivery lines, which I think is the best monitor of all. Put a blank tee in and take a sample whenever desired and see what has actually settled in there. Very small distance between the fitting and the bottom of the sump, so if anything collects, one should see it in a sample. Finally, our annual usage varies from 250 to 350 hours per year. Such fuel turnover is painful to the wallet, but I believe it is good for the tanks, especially if using reliably good fuel sources. For iron tanks, using a magnet on the Racor bowl to look for metal fines is also a good periodic check. FWIW.
