**Be careful about going after oil derrick too early. While they will no doubt pay for themselves many times
over the cost of the infantry and of purchasing the Capture Building upgrade, it can put a damper on your
production capabilities. Build the infantry first and send one to each oil derrick you want to capture. Once
it looks like you will be able to capture it buy the upgrade so once it's done the derricks can be captured
immediately. Doing it this way also allows you to a $1,000 refund by canceling the upgrade if you need the
money immediately such as if the enemy attacks very early. You don't want the money to be wasted on
an upgrade you never had a chance to use.