This paper aims to study the effect of the breakdown behavior of mineral oil, synthetic esters, and natural esters in semi-uniform field electrode arrangement with a realistic degree of homogeneity of 0.2-0.3 under lightning impulse (LI) voltage with positive and negative voltage polarity. Furthermore, different stages of ageing will be analyzed. Finally, these results will be compared to the AC-breakdown voltage in uniform field according to VDE 0370-5:1996-03 to ascertain whether the easy to determine AC-breakdown voltage can be used to also compare the ageing behavior of the liquids.