Just to toss in my ten cents, I used the STP Synthetic Motor Flush and it seemed to work. The area under the oil fill cap was covered in varnish and afterwards, it was gone. I've since put 4000 miles on the car and haven't had any ill effects.
https://www.autozone.com/motor-oil-...er/stp-synthetic-engine-flush-15oz/943125_0_0
https://www.autozone.com/motor-oil-...er/stp-synthetic-engine-flush-15oz/943125_0_0
