Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: Two identical lamps contain the same amount of different types of oil. They are allowed to burn until all the oil has been burned. Which measurement will best provide the total difference in chemical energy between the two oils?
time it took for lamps to use oil
The masses of the burned fuel from each lamp.