A quick update, comments, and a few questions:
Regarding my bleach staining test in my original post, it occurred to me that having the muck saturated with diesel fuel might have prevented the bleach from actually reaching the fibers (due to its insolubility in diesel fuel), and thus skewed the results. So I allowed the sample to dry for a few days and applied some more bleach to the dried sample. There's still no change in color, so I think the sample has at least some asphaltenes.
Out of an abundance of caution, a few days ago I ordered some fresh Biobor, since my current 16 oz bottle is of unknown age, but it's likely >10 years old. During my boat's late October journey to winter storage I was using untreated diesel fuel, so yesterday I treated the ~5 gallons of fuel remaining in the tank with 1/2 tsp of Biobor and 2 tsp of Star-tron Enzyme (with about a quart of diesel fuel as chaser). I have no way to mix this up with the boat on the hard (unless there's an earthquake), so I'm letting it sit for a couple of days and then will pump it through my fuel loop using the manual pump on my Racor filter. After that hope I can get access to my fuel tank to pull off the gauge sender and reach in to pump out the fuel tank contents for storage at home, where I'll inspect it and decide whether to dispose of it or try to manually filter it.
The discussion here brings up the valuable question of what "cocktail" of additives we should use for protection against bacteria, corrosion, water contamination, and lubricity replacement for newer ULS fuel. Pre-treated Valvtect fuel is no longer available around here, so I need to find additives. (Note also in the attached report that Valvtect Diesel Guard addtivite actually makes lubricity worse
than untreated ULS fuel.) And, FWIW, I buy all my fuel at busy land-based fuel stations because I always think that they are less likely to have water contamination than marinas.
The discussion of using multiple additives is potentially complicated because of the potential for them to attack each other with competing chemistries. In another post last fall
I referred to the old Steven Wright joke about putting a humidifier and a dehumidifier in the same room together.
For the first year with my current boat, I used Biobor JF and Star Brite EZ Store EZ start, which were the two additives that the previous owner left behind. Then, after reading the well-known lubricity additive study
(see attached picture), I replaced the EZ Store with Opti-Lube XPD, which was rated very high for lubricity properties. I was concerned that the two might interfere with each other because EZ store says it's an emulsifier, while Opti-Lube says it's a demulsifier. I've run out of the Opti-Lube, so will either order more for spring launch or find something else based on recommendations that I get here.
After re-reading the Practical Sailor articles, I've added the StarTron Enzyme additive for corrosion protection in addition to the BioBor JF. These seem consistent with Practical Sailor's recommendations, though I'd appreciate comments on whether to re-order the Opti-Lube or find another lubricity additive.