Perhaps this is well known issue to most of you, but it is new to me. So hopefully this helps someone.
I did an oil & filter change on my 2008 3720 this weekend. Also changed the fuel filter, and cleaned the sediment bowl. I noticed some slime in the sediment filter and got concerned. After cleaning and reassembly, and starting the tractor, the fuel appeared milky, and I could not even see the red ring in the bowl.
Next, I drained fuel tank by removing the hose and screen at the bottom of the tank. Screen was full of snotty slime and fuel drained slowly. I inserted about 18 inches of a braided cable into that drain hole cleared a bunch of slime. Fuel then drained as expected.
I bought some Bio Kleen at Tractor Supply, and added it to 5 gallons of new clean (highway) diesel. I used the sludge / shock proportion. Reassembled cleaned drain screen, fuel line, ...etc and refueled with the treated diesel. Took a rigorous drive down gravel road to slosh it around. Fuel looks clear in the sediment bowl.
Letting tractor sit till next weekend. I’ll be adding Bio Kleen to all future fill ups. I hope I’ve dodged a bullet.