Try some additives Like Lucas Upper Cylinder Lubricant, Power Service
Silver, Howes and keep some Algeacide (or occasionaly up to 1% gasoline will also accomplish this) product in your fuel
. Other good products I would recommend are the Liqui Moly 2037 Pro-Line Engine Flush at each oil change
, Liqui Moly 2002 Super Diesel Additive - reduces friction, Liqui Moly 2005 Diesel Purge - 500 ml (watch youtubes on how to use this). All available from Amazon.
This new low sulpher diesel fuel
needs help and the older motors hate it. You need to add something to increase Lubricity and keep your rings free and decarbonded.
I had 28,000 hours on a 2 cylinder Yanmar
on my Semi-Trucks APU (auxiliary power unit) that ran HVAC
and kept batteries charged when not idling the 15 liter cummins main engine at night, it was serviced evry 600 hrs and did use oil but still started and ran fine.
I rotate all these products on my 1982s diesel mercedes 300CD and mercedes powered 300 Turbo diesel Toyota 4x4. The problem with most sailboats is the lack of hours these motors get in extreme conditions.
I would not recommend synthetic oil for oider motors as the advatange of extended interval change is lost
unless your running lots of of hours (which you don't on boats), and oil consumption
will increase and compression will decrease with the thinner oil. Your oil should be changed a minimum of annually or the recommended hours by manufacturer.
On our farm tractors this was 100 - 200 hours depending on how dirty the use was on over the road Semi's its about every 500 hours or about 20,000 miles depending on idle time, more idling shorter interval.
Diesel Mechanics aren't cheap
and often do things that aren't required (hey they need to make a living too) when some good preventative practices can keep them out of your pocket.