White goo on oil filler

[Louis]

I've been noticing a white slim/goo around my oil ser. tube and on the oil cap when I add oil.
I norm. drive it twice a week, each time I go about 10-12 miles about 35-40 min.s to make sure the temp. get's up to normal before I park it.
I use 15W40 Rotella oil plus about a 1/2 abottle of STP on oil change, and I change it when it get's dirty, about every 300-500 miles, all town miles 35-43 mph avg.
The rad. seldom need's servicing, their is no water in the oil. and the eng. runs ok.
Any ideas why ?......Thank you.