My prefilter had a decent amount of black alge(?) in it. What was impressive was the spin-on filter. I emptied it into a jar and couldn't see light through it! Totally black, with some floating clumps!

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1983 300td 240k (down with bad tranny)
1984 300d 222k (daily driver) "Olive"
1997 GMC K2500 105k (sled dog limo)

"The scenery in ANWR is as spectacular as the Grand Canyon and the wildlife is more impressive."