The little black specks are most likely asphaltenes, a part of the diesel fuel degradation process. They can clog things up. This article explains the process pretty well.

Source Of Diesel Fuel Problems

When I've gotten new to me older diesels that have sat for awhile, I like to change out the fuel tank strainer, primary plastic fuel filter, and metal secondary fuel filter.

I also replace all the old rubber hoses with Viton hoses.

Then I'll do a diesel purge with a quart of ATF, three or four days in a row with a mini-tank letting the ATF stay in the IP overnight.

Then I run regular diesel number 2 with Startron diesel fuel tank cleaner (available from West Marine or other boat supply store) for a few tanks replacing primary filters as they clog.

I then switch to B20 for a few tanks and end up with B99 for a few tanks, replacing filters as needed.

This really cleans everything out. Then I switch to my favorite fuel in the Bay Area, Propel Diesel HPR or Diesel Number 2 from high volume truck stops when out of the Bay Area.

As you bring your car into regular service, it's a good idea to carry extra fuel filters, a mini-tank with hoses and the tools to install them.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Lots of good reading on that site! But then I noticed they sell "magnetic fuel conditioners"

I thought that theory had already been put to rest, no? Curious what you (and others) think about magnetic fuel conditioning

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2007 Dodge Ram 3500 Cummins Turbo Diesel 4x4
1994 GMC S-15 pickup 4.3 5MT
1985 300 SD
1978 300 CD
1962 220-S Fintail - awaiting restoration

I think the rationale behind the use of magnetic fuel filters is that since iron is an essential nutrient required by microorganisms (and us) by removing iron from fuels you inhibit the growth of microorganisms that are present and slow the inevitable clogging of the regular fuel filters.

In theory they offer some benefit, but I doubt we need to use them on our cars.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022