I've wandered this myself, I was using Mobil 1 Extended Performance for the last year or two. Although I wasn't able to bring myself to lengthen the oci's very much, I just changed it out cause at five months old and less than four thousand miles it had used some oil and it was darkened quite a bit. Which I am not use to since I bought the car new and have been "wasting money" doing oci's every three months "per warranty" with only usually 1-2 thousand miles on it. With mobil 1 synthetic since 2,000 and only 28,000 easily driven miles,I didn't think it should have either.
I know, I know
Putting on flame suit!