What made transformer oil unsuitable for further use

MENTION FOUR (4) CAUSES THAT RENDER TRANSFORMER OIL UNSUITABLE FOR FURTHER USE [4

1. Low electric strength – may result from many causes, the most common is foreign particles on fiber and water in combination.
2. High acidity
3. High sludge content
4. Excessive free water content – presence of moisture can be detected with oil dryness test set.