90LS,
I suppose it's possible the sludge is from the infamous PS rack leaking - you know, spitting onto the oil pan under highway speed.........that is, if you've ever had the PS leak before???????
But really, your LS is 14 years old!!! That gasket has got to be cracked in a few pinhole places - all those years of heat/cold, etc....
It's never the miles that get the LS down.........its the age