There have also been very good used oil analysis here at BITOG with regular Pennzoil 5W-20. Not sure about Castrol GTX... don't recall seeing any with it in 5W-20.
I ran it for 1k after I had the factory fill in for 4k. That was almost a year ago so I don't recall how it ran. One thing I remember though is that after 1k the oil was really dark. Maybe I'll try it again after I finish my mini-stash.
In reply to someone's question about the 10k oci interval for a Civic, I run my ocis for 5k. Maybe when I drive more miles in the future I would run 10k on synthetic but I don't think I will ever try that with a conventional oil.
Sometimes the store only has it in the back room cases -- other times they will special order for you & it arrives with all the other Havoline weights on the store's regular oil semi-load day.
I know which day all the auto part stores around here take delivery of oils from their warehouse.
I plan to... I replaced factory fill with Havoline 5w-20 in my '05 Civic VP and my intent is to run a 10k OCI assuming my driving conditions remain the same (I ran my factory fill to 10k and pulled a sample for UOA).
Based on my factory fill UOA, Blackstone recommended pulling my next sample @ 5400 miles. I'll do that and then adjust my OCI accordingly.