I am just running into this as well with HTO-06
I've been digging and digging and I think this thread has INCORRECT information.
I cannot find any specification that superseds the HTO-06. One person is correct in that it was GF5 testing plus the hot tube test on used oil. But without that part of the test it's not superseding HTO-06 and apparently it's hard to pass or is cost prohibitive.
I will give some case in point examples that apply to me and this thread.
First it is important to understand that I have not found any oils that are not 5w30 that bear HTO-06 certification.
Secondly when one is found like Mobil 1 or Pennzoil Platinum, if you actually look you'll find that it's high mileage brother does NOT bear the HTO-06 certification. But yet they have the other certifications including GF-6. So I don't think GF-6 oils surpass or supersede HTO-06 like SP does to SN.
I don't know if it's just cost prohibitive for a certification that applies to few turbo cars that is probably not even considered by many drivers, or if there is something in the high mileage add pack that causes it to fail the test?
I'd sure like to know because I for one already made this mistake and didn't catch it until I the first time I pulled the dipstick and checked the oil level and saw it was already as dark as it is by the drain interval. I started head scratching wondering what the heck changed so I went comparing bottles I keep to carry my used oil away and noticed I had just assumed the high mileage version had the certification since the regular did. Doesn't seem to be the case.