I've seen this in the winter time, when the temps are below 20. If it's the gray milkshake looking crud up in the hollow of the cap....
My experience has been that all of my cars and trucks (dohc v-8, honda 4 cyl, inline iron block 4, v6,) will deposit this goo in the deep wintertime. My guess is condensation not getting cooked out of the crankcase, or forming in the cold recess of the cap or filler neck.
I wipe it out with a paper towel. Look again in a few days, and you'll see water droplets forming. Won't happen in the summertime, or even when ambient temps are above freezing.
FWIW, I did lose a head gasket on a car and had coolant mixing with the crankcase oil. It was a similar substance, but it was on the dipstick, in the oil when changed, the oil was gray, with microscopic bubbles throughout.
This is not that, if this is what it is.
