Diesel Fuel Additive

Microbes like mold, bacteria, and fungus are your black stuff in the tanks. Seems to me that asphaltenes can be best handed with injector cleaner (which is another fuel additive (snake oil to some but useful where a problem exists). But I think this sounds like it would be more of an issue with high use vehicles like trucks or commercial ships rather than low use vehicles like recreational boats. Microbes on the other hand love not being disturbed.

It's more than just a little fuel returning to the tank in most modern engines. Keep in mind that the flow fuel is a large part of lubrication of the injection pump and it keeps the injectors cool as it flows through and returns to the tank. Most larger engines have fuel coolers for the return fuel going back to the tank because otherwise the tanks would become very hot after a long run. Hot fuel degrades performance. Common Rail engines take the pressure of fuel up in the 20-30,000psi range and then don't burn but a small portion of it that flows through the engine. Anytime you build that much pressure, heat is very much a byproduct. Put your hand on the pump of a pressure washer after it's been running for an hour. In addition, tiny, very precise passages and incredible pressures need good lubrication and thus fuel additives have been developed to provide better lubrication.
Keep in mind the refinery process. Crude oil is heated. Asphalt is at the bottom of the tower, fuel oil a little above that, diesel, gas, and lighter stuff towards the top. Asphaltenes are the fuel being taken to extreme heat and breaking down into components.

You're correct in what you say. I just question how big a problem it is in motors that are only run 100-200 hours per year as opposed to the thousands of hours put on truck motors. If it were that big a problem with as little use boats get our fleets of trucks would have ground to a halt long ago since use is what causes this issue. Of more immediate concern is what clogs it from just sitting around since that's what most boats do most of the time.

I live in an extremely warm/hostile environment (Darien Gap). All diesel fuel has water. Make sure the water-separators on the fill pumps are new and if the marina can't tell you - then make sure the ones on your engines are new. Add a bottle of Stanadyne, Lucas or Power Service. The newest fuel (ULSD) has most of the sulphur removed and very little lubricity left in it. So by adding Lucas or Howe's you are adding lubricity. We do that at every fill (600 gal).

I'm simply guessing, but most trucks are running full throttle occasionally and the engines are varying their loads including lots of running at heavier load conditions. Maybe they burn more fuel overall instead of running lightly loaded and returning more to the tanks. Most boats spend a lot of time idling and cruising at lower rpm and then hanging out in the engine room checking and obsessing over filters.

I don't think truck loads or how much throttle is used can be generalized. The only think known is that time is money. If your truck is parked you don't need a truck. Show me a truck that only puts on 200 hours in a year and I'll show you one with a for sale sign. Recreational boats on the other hand spend most of their time parked, motors off. Their fuel isn't being pushed through the injectors. It's sitting and growing stuff.

Why go looking for trouble?

There isn't any point where a heavy truck should run light. My Dad was in the Army heavy haul divisions for 30 yrs.
His M35's hauled five to ten tons constantly with 371cid burn anything engines. Today's diesels are more finicky because of turbos = which the older engines didn't have. And they ran perfectly well. You could work on one without having an electronics/engineering degree.

Whether trucks run heavy or light they're running constantly. Recreational boats barely run at all. That leads to completely different problems.

Many Army’s still use Detroit Diesels in land vehicles today, as they get government exceptions on today’s emission standards. It is easy to design simple mechanical Diesel engines, but a it is a whole other topic to design one that meets current emission standards.

The last truck I drove had a Detroit 60 and 18sp tranny. Got better mpg than anything currently on the market.

I love the Series 60, was able to install the first of 3 pre-production units in a yacht.

But they lost the battle on two fronts - after a hugely successful and life saving run, Detroit Diesel moved away from the platform on its on-highway business and it lost its volume advantage and the emissions rule finished it off for the most part.

The JD and DDC joint venture era was interesting and political at the same time. Penske is a large and in charge kind of guy, so those types of corporate cooperation ended when it was deemed less than beneficial to one of the parties. The S60 design was a bit long in the tooth compared to the competition and the truck/cab builders were driving change.

On highway needs did not mesh for the future models off of Series 60 and replacements became taller than usual which is a deal killer for boat applications. There was a furious on-highway emissions battle with alliances being done with truck builders, much like what we have seen with Mercury and Brunswick, buying transom so to speak.

S60 was king at one time, the beloved million mile engine, then CAT got some market share and eventually Cummins won the on-highway battle with a surge by Volvo Penta. It can’t be understated how the on-highway volumes benefit the business case for Marine applications, without the volume you just have dreams.

My little old computerless DD J&T 471-ti's just keep on running and sip fuel at 1550 rpm making 10/12 knot depending on the current.....
I'm not really sure of the burn rate though..Lol. But they just do...., burn fuel. I think I will keep them.
Oh , to keep the fuel clean. I use/run the boat!