I believe you are referring to a new Phillips oil that the company named X/C II, as an advanced version of the original multi-weight X/C.

The new oil was among the first to have an anti-wear additive and there was no mechanism to approve the oil. There is a mil spec for oil, but it hasn't been updated in decades so it was of no use. The other approval route is to have each engine manufactutrer test and approve an oil but at the time Lycoming and Continental were at a low ebb in business and lacked the resources to test and approve the new oil.

As I recall, Phillips paid a fleet operator to conduct enough flying on the new oil to get an FAA STC approval, but it was not a conventional approval from the engine OEMs. Some FAA inspectors insisted that operators had to have a Form 337 because of the STC before the oil could be used in each type of engine, particularly for a 135 operator. Hangar tales have it that some FAA inspectors demanded that 135 operators fill out a new 337 every time they changed the oil, or even added a quart. Confusion grew out of this and soon everyone was wondering about the approval status of X/C II.

I used the oil in a Bonanza I had at the time with good success, but there was so much controversy over the oil that Phillips finally gave up and withdrew X/C II from the market. I don't know of any flaw in the oil that was ever demonstrated, but once the controversy and confusion began there was no way for Phillips to recover.

Mac McClellan, Flying