| Good Oils | Proposition A Oils | Marginal Oils | Bad Oils | Very Bad Oil | Extremely Bad Oils | Disastrous Oils |
| 1 | 2 | 3 | 4 | 5 | 6 | 7 |
| Pale Yellow | Yellow | Bright Yellow | Amber | Brown | Dark Brown | Black |
| Effect on transformer | ||||||
| Proving these functions 1) Efficient cooling 2) Preserving insulation | Polar compounds (sludge) in solution (products of oil oxidation) causes the drop in IFT | Fatty acids coat the windings. Sludges in solution ready for initial fall out. Sludges in insulation voids highly probable | In almost 100% of the transformers in this range sludges are deposited on core and coils. Sludges are first deposited in fin areas. | Deposited sludges continue to oxidize and harden. Insulation shrinkage is taking place. Premature failure a good possibility | Sludges insulate cooling fans, block vents causing higher operating temperatures | Vast quantities of sludges may require other means than desludging procedures. |