One refinery makes/processes 500 bbls. per cycle. The tank holds' 500 bbls. Since The cycle runs like this:
:10- Oil produced (appears in tank)
Five minutes later,
:15- Oil turned into kerosene (If being refined)
and to complete the cycle, in another 5 minutes
:20- Kerosene sold.
If you put in 300 bbls. of crude, It will be refined and sold before any more oil comes into the tank. This is if you get one refinery for now. If you get a second refinery, You will need to upgrade your tank again, because even with still only 300 bbls. of oil in, you now have an extra refining step holding oil in the tank, so your production ladder after the :10 after Oil-in cycle would look like:
300 BBls Crude oil.
300 BBls. kerosene.
300 bbls. Heating oil
=900 bbls.
This is why tank size is important, but in order to get the max amount of money coming in, without wasting well prod. is to get a refinery now, refine everything you make once, make more money for the same amount of time, and then upgrade your tank and Dist. center to handle more output, and then start to build more refineries'.
Always make sure you have enough cap. on the other end of the production train to handle everything you make, or extra pumps are wasted, due to not being able to sell everything you make, or hold everything you make.