The oil cap deposits are sending you a message.

Modern oils contain an additive that immulsifies any moisture into the oil preventing it from depositing this moisture throughout the crankcase. When the oil has immulsified all the moisture it can contain, extra moisture is deposited around inside the engine. What you are seeing on the cap is a small portion of the moisture being deposited all over the inside of the crankcase.

It is time to drain the oil while the engine is at full operating temperature. It also is telling you that you need to change it more frequently.

Best of luck,