Dirty oil, part II

Back in January, I posted my concern about the oil in my 172RG (Lyc. O-360)
turning black just two or three hours after being changed. My regular
mechanic said no to worry, the crud probably came from oil that didn't get
drained from the oil cooler and associated plumbing.

Still concerned, I took the bird to Teledyne Continental's shop across the
Bay in Fairhope for an evaluation of the engine. They reported that
compressions were good, crankcase pressure was ok and borescope examinations
revealed nothing unusual in the cylinders. Relieved, I concluded that my
mechanic was probably right. I resolved not to worry about it.

Now I've got another airplane with an engine (Lyc. TIO-540) that is very
similar, at least in its cylinders and bottom end. I had the oil and filter
changed as soon as I got it because the oil was black. 15 hours later, the
oil is just barely showing enough contamination to make the dipstick easy to
read.

So now I'm wondering: what was really going on in the first airplane's
engine? What could have been getting the oil dirty so quickly yet not show
up in the TC shop's examination?

--
Dan
T-182T at BFM