The 81 300D recently purchased mostly sat the over last 10 - 15 years, perhaps longer. I have been going through and replacing and repairing as I find issues, and am pretty close to it, and it is now my daily driver.

One issue that was identified recently as as algae or something biological in the small fuel filter, it also seems to have affected the fuel sending unit.

What would be the best recommendation for an additive that would kill these pesky organisms?