D4 7u oil in fuel tower??

I decided to drain all of the old diesel out of my d4 7u including the tower. Plan to change those sock filters in there. The thing is that I pulled them out and found old black oil in there underneath them. There wasnt any on the filters at all but there is near the bottom and in the bottom of that tower. When I loosened the plug to drain, it drained probably about a cupful out of there and then the diesel drained. I've attached pictures because I couldn't find this question being asked anywhere. Is this normal or what's wrong if not?
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Its most likely old paraffin wax from old diesel fuel that settled out or it could be the seals on the transfer pump let oil seep in over the years. At the base where it mounts.

Sounds like excellent Cat engineering for that era to provide both a reservoir for "stuff" to accumulate and a drain to remove it.

That's normal for there to be brown sludge in the filter housing like that, and that much is not bad at all, the filter system is doing it's job.

Ok thanks guys! I'm cleaning it out good. Hopefully fresh diesel will make a difference in cranking it. It's harder to crank than it should be so far

On another note, oil also works as fuel.

As long as the fuel filter does its job, you can basically pour your used oil or almost any oil into your fuel tank to fuel your tractor. Vegetable oil, peanut oil, engine oil, probably even crude oil.

However, oils tend to be a bit thicker than diesel fuel, so depending on temperature and the oil you use you may need to add some heat. Or just adding some oil to a tank of diesel fuel would be okay as the diesel would help thin it out.

I also hear of people mixing gasoline with their used oil before putting it in their fuel tanks.

So, oil in the fuel system isn't necessarily a bad thing unless it is taking it from the engine lubricating system and using it up.