Conversation so far:
Who was in a business partnership with John D. Rockefeller?
In 1859, John D. Rockefeller went into business with a partner, Maurice B. Clark.
What kind of business were John D. Rockefeller and Maurice B. Clark into?
John D. Rockefeller and Maurice Clark went into the produce commission business that transitioned oil refineries.
What other business partnership besides Clark's did John D. Rockefeller have?
In 1866, William Rockefeller Jr., John D. Rockefeller's brother, built another oil refinery in Cleveland and brought John into the partnership.
What else can you tell me about John D. Rockefeller that is interesting besides his partnerships?
The firm established by John D. Rockefeller, Andrews & Flagler was the predecessor of the Standard Oil Company.
What did John D. Rockefeller's Rockefeller, Andrews and Flagler firm do?
John D. Rockefeller's firm refined petroleum and marketed by-products of the refining process.
When did John D. Rockefeller buy out the Clark brothers?
In February 1865, John D. Rockefeller bought out the Clark brothers for $72,500 (equivalent to $1 million in 2018 dollars) at auction and established the firm of Rockefeller & Andrews.
What else did John D. Rockefeller do about oil besides sell it as fuel?
John D. Rockefeller used gasoline to fuel refineries, selling by-products like lubricating oil, petroleum jelly, paraffin wax. Tar was used for paving and naphtha was shipped to gas plants.

Question: what did they use when whale oil got too expensive?
Self-contained rewrite: What did John D. Rockefeller use when whale oil got too expensive?