Gunk on the oil cap.

[Kevin Wood]

The appearance of the mayo is just a sign that there's been water vapour in the crank case. Without the filler neck, it gets washed off by oil flung onto the back of the cap by the camshaft. It doesn't in any way change the conditions inside the engine, and wiping that small amount of may off with make no difference.

The main thing to do, especially if doing lots of short trips in cold weather, is to make sure the breathers are clear and keep oil changes reasonably regular.