How come the melting point of diesel petrol and rapeseed oil is only approximate?

Well I wish you could have choices but I look it up Diesel fuel originated from experiments conducted by German scientist and inventor Rudolf Diesel for his compression-ignition engine he invented in 1892. Diesel originally designed his engine to use coal dust as fuel,[4]and experimented with other fuels including vegetable oils[5] such as peanut oil, which was used to power the engines which he exhibited at the 1900 Paris Exposition and the 1911 World's Fair in Paris. Maybe that will help your answer.

Tolerance to high levels of salinity