Answer
Mixing incompatible residual fuel oils (ISO 8217) causes asphaltene precipitation, creating a heavy, sludge-like substance that instantly clogs purifiers, fuel filters, and fuel pumps, leading to a total blackout.
The ASTM D4740 Lab Test: The standard laboratory test for compatibility is the Spot Test. A drop of the blended fuel mixture is placed on a specific grade of test filter paper and heated to 100^C.
Evaluation: The resulting spot is inspected against a reference chart. A uniform, clean spot indicates compatibility. A dark, distinct outer ring or a heavy black sediment cluster in the center indicates asphaltene separation (Incompatibility).
The Shipboard Practical Test: Since you cannot always run a full ASTM test on board, the crew uses an automated portable onboard compatibility test kit (relying on the same spot principle) or executes a bunker segregation policy. New fuel must always be loaded into empty storage tanks; never mix new fuel with existing fuel on board above a strict 80/20 ratio unless a compatibility test has confirmed it is safe.