Dirty yellow/orangey gunk underneath oil cap.

Recently I drove my car for the first time in two weeks, after a short drive, the oil light came on. I checked my levels via the dipstick. They seemed normal, so I removed the oil cap. Underneath there is a thick dark mustardy coloured substance evident. I have no mechanical experience so have not yet tried any solutions in case driving or tampering causes damage. Is it possible to diagnose from this? And further price potential repairs? Thanks for any assistance!

UK.