How can something be five times lower, or five times less, or five times smaller than something else?
Clearly, the people who write such expressions do not understand fractions, decimals, or percentages (although they might comprehend factors or orders of magnitude). Petroleum engineers, however, should exhibit ample evidence in their writing that they eat fractions, decimals, and percentages for breakfast. Therefore, they should never use these expressions, lest someone think the other numbers in the document equally fuzzy.
In the engineering world, five times larger means 5× or “multiply by 5.” If X is smaller than Y by a factor of five, then X = Y/5. That means X is one-fifth the size of Y, or 20% of the size, which translates to 80% smaller or a value that is 80% lower. Use those expressions instead to ensure clarity and give the impression that your number sense is 100%.