​I was frustrated recently.  There were only a few oil locations on the entire map, and I unfortunately was unable to access them or trade for them.  I discovered new technologies that made ironclads, mortars, and cavalry all obsolete.  Unfortunately, I still had a number of pre-industrial units like carracks and Men o'War, mortars, cavalry, etc.   Since the most recent upgrades now needed oil, I was unable to upgrade these units at all, not even to their industrial age counterparts.   My cogs and Carracks could not be upgraded to Steam Frigates, and my Men o'War could not be upgraded to Ironclad even though Steam Frigates and Ironclads don't need oil.   The only options available to me were the contemprorary era units.

My recommendation is that we have the option to upgrade to and even build less modern versions if we desire.

Yes, Invading those locationa or forcing an alliance are ways to get access to the oile in this case, but getting access to the oil wasn't the issue.  Not being able to upgrade to the latest pre-oil unit is the issue.