My personal experience of rainforest is confined to Malaysia, where it is too damp to burn unless it is first cut down and allowed to dry out in the sun. The fires on these ladangs, as they are called, never spread to nearby rainforest, and abandoned ladangs eventually become rainforest again. I have some knowledge of other rainforests and have come to the conclusion that true rainforest is similar to Malaysia's. Throughout the wetter parts of the tropics, where there is not rainforest there is a kind of forest called monsoon forest which is only wet for a few months of the year. After that it dries out and eventually loses it's leaves as though it were winter.
Often it is fairly close to real rainforest, so there is a gradual transition from monsoon forest to rain forest. Both kinds are found in Amazonia and Central America, Dropping 5,000 gallons of gasoline into a rainforest to see if it would burn would be regarded as very odd nowadays, but it frequently happened in WW2. Allied and Japanese aircraft were frequently shot down and crashed into the jungle, seldom with 5,000 gallons of fuel aboard but certainly enough to cause a fire. In Malaya and Borneo the jungle never caught fire, and although this happened all over S.E.Asia, surprisingly enough I have never heard of an incident where the jungle caught fire. There is some monsoon forest in India, but the fighting there was confined to Assam, where it is rainforest.
In modern times a large jet occasionally crashes into jungle, but again I have never heard of one setting he jungle alight. Jet fuel, of course, is not as inflammable as gasoline. Some of the fires in Brazil currently shown on TV look as though it is the sort of scrub which springs up on land which was once cultivated.