Now here's a story for those of you who think there isn't much oil going down there.
I had cause to sort out an engine that had been rebuilt over a period of time and not very long after startup, spit the oil return line off the bottom of the tank with the resulting spray of oil all over a newly finished exposed aggregate concrete driveway. The owner took the oil tank off and had it cleaned and suspected the oil filter so replaced that which was perfectly intact, and then brought it to me to make sure all was good.
I reasoned that as the banjo on the bottom of the oil tank was plugged with fuzzy stuff that it was on the return side of the oiling system, so I started there, removing all the returns that pass by all the locking rocker feed bolts, and finding bits of fuzz, pulled all the bolts and the rockers themselves, now I can tell you that the fuzzy bits in less than a 10 miles run, had made it down past the jiggle (Joggle) wires and was just starting to show up in the rockers. All that was then taken apart and de-fuzzed! I looked all over the suction side and everywhere else I could think and found no further evidence. I was still contemplating the source, especially as the owner had said that the UFM with all the front end attached has been stored in an unheated garage over winter and the neighbour had left a used mattress close by at the same time. He, and consequently I, assumed that there was a mouse nest in the oil tank. The more I looked the more I was convinced this was not so, as stated before all the suction end was clear.
Eventually my brain engaged and the engine drain plug was removed and a bent up coat hanger went fishing in the bottom end, Sure enough two large clumps of mouse nest, well oil soaked were removed followed by a good flushing with some solvent. after thorough cleaning and check I put a few tenuous miles on the bike, and then had another look at things, no fuzz to be found. The engine had sat at some point unattended....I can only assume the mouse (or mice) left town before the cylinders and pistons were fitted, or perhaps made their escape through the drain plug hole.
Anyway, the thing that stuck with me was how much fuzz had made it past the wires!