When I removed the oil cap the opening and the underside of the cap was covered with a thick yellow gunk. What is causing this , is it a problem, and if so how do I solve the problem.
Do you usually drive the vehicle on very short trips often? If so, that can cause the sludge you see. Otherwise it could be an indication of a coolant leak into the engine.
I’ll speculate that your driving routine is composed of mostly short trips. It can be a design anomaly in the PCV system …or even a head gasket leak.
Assuming it’s your driving style, there really is no solution that makes sense other than changing it out more often. That won’t relieve the symptom in many cases. Otherwise, install a block warmer or drive further. That’s why the solution (assuming the aforementioned issues are not present) is a paradox. Why would you drive further, spending money in fuel, JUST to appease your engine?