When I went to add oil today, there was yellow stuff all over the under-side of the filler cap. Why is this?

I don't know what year vehicle you have but newer vehicles have a closed crankcase vent system, that is, the oil filler cap has a seal to prevent crakcase vapor from escaping except to the combustion chamber. To me that looks like water from condensation. I don't imagine you are having any other engine or cooling problems. Clean the stuff out and replace the filler cap and then check it out after a a coupla weeks, see if it returns.

It's a 99 Ranger with a 4.0. I was having some cooling issues last week but it was fixed. Running 10-40 in it.