Just wondering if anyone could tell me what is causing this brown goo to form underneath my filler cap

I have done a compression test and all if fine. Oil is fairly new Valvoline. No smoke blowing. This one has me puzzles me.

Could it oil break down?

Check your dip stick to make sure that the same isn't occurring in the sump. If your dipstick is fine don't worry, it's just water vapour collecting under your oil filler cap. Take your car for a long drive and all shall vanish.

If it's in your sump it means you might have issues and is worth investigating.

Nothing on Dipstick. Thanks for post.

yes no worries....just condensation.
It would be another story if the dipstick showed the same (head gasket leakage)

John