Was there any UV exposure during storage?

It was exposed to sunlight for about a week at room temperature

Does the sample contain any Hindered Amine Light Stabilizers (HALS)? If so, the possible antagonism between the HALS and PAO (for example your BHT) could have rendered some of the latter ineffective.

These are quite helpful answer, many thanks. But I just figured out that that sample that I tested long ago was actually without BHT in it.

But thanks anyway.