Q.What is the difference between persistent and non-persistent oils ?

The need to differentiate the oils as persistent and non-persistent was because we need to know which oil are difficult to clean. 

Civil liability convention 92 and fund convention 92 applies to the pollution involving persistent oil only. Why so ? Simply because persistent oil leave a thick slurry of oil on water and are difficult to clean or remove. 

Or persistent oils break up and dissipate more slowly in the marine environment and usually require a clean-up operation. Crude oil is one example of persistent oil. 

Non-persistent oils will dissipate easily and quickly through evaporation and so these oils will not require an active response to clean up.  

This is more of subjective definition. The more precise definition is given by the EPA as per which

Non-persistent oils include:

1. A petroleum-based oil that, at the time of shipment, consists of hydrocarbon fractions at least 50 percent of which by volume, distill at a temperature of 340 degrees C (645 degrees F); and  at least 95 percent of which by volume, distill at a temperature of 370 degrees C (700 degrees F); and  
2. A non-petroleum oil, other than an animal fat or vegetable oil, with a specific gravity less than 0.8. 

Persistent oils are oils that do not fall under above category.

Because carriage of non-persistent oils have comparitively lesser risk, the P&I clubs in certain conditions allow certain premium return if the ship has carried non-persistent oils.