For some reason there isn't a page for oil (basilosaurus) so i'll write one here:

they can produce 20 oil at a time

convenient cuz you can use the oil to make an okay amount of gasoline (15 per 20 oil cycle), and then the oil that doesnt spoil (for example the ones you get from harvesting basilos) can be stored somewhere else for later

If you have both a basilo and a tuso you can collect the oil from both of their inventories to get 90 oil, which will make 75 gasoline and there will be no remainder oil left to spoil.