Those toilet paper filters are called Frantz filters (still made), and I used to have one on my old 70 Chevy C-20 big block for years.
I used to change the oil on my old Pathfinder diesel engine
every 100 hours (in my boat), but I change the oil every three months or 100 hours depending on what comes first. The reason I changed the frequency of oil changes is based on information I got from a man that used to be in charge of motor
pools in Desert Storm, and he used to have his own automotive repair shop, and taught mechanics around the world on new changes as a teacher for the Chrysler Corp.
He told me that the longer the oil sits in a diesel engine, the more acids form as the oil mixes with diesel fuel
. As the acids build up in the engine it slowly eats at the engine, causing damage to parts
and block over time. As he mentioned to me a sailboat may take a very long time to add up a 100 engine hours of time, so the longer that contaminated oil sits in the engine the worse it is for the engine. His recommendation was to change out the oil at least every three months, whether the engine had 100 hours on it or not.
You can do your oil changes whenever you would like, but this is what I do now.