Is the dipstick picking up some condensation in the dipstick tube? Change the oil just because it's a good idea anyway, but run a litte fresh oil down the tube to rinse it out, then see if you still get the same results.
This is exactly what I was thinking... Did the sludge show up as soon as you removed the stick the first time? Or did you remove the stick and wipe it clean then reinserted it to check the level?
It sounds like you may be getting some steamy oil up into the tube. If your oil temperatures are running cold for long periods of time the water never boils out of the pan.
One really cool way to fix this is with an oil cooler (hang in there for a second.)
I have commonly installed oil coolers on jets with over transom headers to try and keep the oil temperatures more consistent. Install an oil cooler using the engines outlet water as a cooler feed line, this water is typically very warm at idle/lower RPM (should be in the 160 degree range) and cooler at WOT, more often then not, in the 120 degree range if you think about it, this is perfect for keeping the oil temps under a pretty close range.
Good luck, sounds fairly normal to me.
GT
