Is your minivan warming up as it should? A thermostat that is stuck open will not let the engine warm up and this can cause the gunk you are seeing under the oil cap. If the temperature is coming up as it should, then short trip driving may be the cause. It used to be that some brands of motor oil seem worse about having this gunk build up than other brands, but I don’t know if that is true today. If you changed brands of oil, that may be a possibility.