Is Castrol Ultra Clean a full synthetic oil? If so, group III? Group III+? It sounds like a great oil that may be ideal for this purpose.
I am finishing an OCI of Pennzoil Ultra Platinum in our 04 Jeep 4.7. It ran conventional with the previous owner up to about 106k miles. Just looking at what I can see of the valve train, it has done a tremendous amount of cleaning. It also stopped mild oil usage. I've been pleased, so far. I'm planning to run at least another 3k to 5k OCI with Ultra Platinum (5w30, btw) in it before settling in on either Mobil 1 or Pennzoil Platinum for the rest of its life.
All of my engines over the years that have used Mobil 1 have been incredibly clean and free of deposits, including an 06 Toyota 4Runner 3.4 that was known for sludging. It looked new under the valve covers at 130k or so when Toyota inspected it. The Toyota service tech asked me what I was running and when I told him, he said, "I thought so." That was years ago, before the group III's came around and were called full synthetics.