I think, based on using Amsoil for way to many years, there 35K miles is not gonna happen. There EAO filters are the best out there and 20K is not out of reach (25K is getting out there). I would use there normal oil unless you need the extra ZDDP, moly and or TBN.
Based on using it and testing, 15K would not be a problem (with most of there oils). Neither should 20. I do not know the engine history but I would test at 12, 17, and 21. Problem with extended drains is if you have poor gaskets, poor air filter, poor fuel system they all feed into the oil (and tests will show).