Lightning Impulse on Transformer

What I know is that oil breakdown voltage under negative polarity impulse is always higher than for positive impulse. This means, transformer should be subjected to a higher level of voltage if tested with negative impulse. But, this is only IEC's approach. Some other standards use both positive and negative impulse for OIL filled transformers. Also check this article: http://www2.nynas.com/naph/start/article.cfm?Art_ID=756&Sec_ID=55