If you need small amounts of crude oil...
     ...and you don't have centrifuge:
               -buy few amounts from people's shops
     ...and you have centrifuge:
               -get few coconuts from starter's beach (2 coconuts for 1 crude oil, 4 cocounts for 2 crude etc.).
               -Make Coocunt Oil from coconuts using centrifuge tool.
               -Finally recycle coconut oil in trash bin until you'll get crude oil.

If you need large amounts of crude oil:
          -build or buy Centrifuge tool
          -build toilet or buy many stacks of organick muck (though buying not recommended)
          -stay overnight on remote on any monster droping fruits. I recommend beetles in the last room of beetle cave, south           island- 6 beetles respawning there :)
          -If you have another char, give him toilet and stay overnight doing organick muck. You can also make it with main           char, after or before gathering fruits.
          -Now you have plenty full stacks of fruits, and plenty full stacks of organick muck.
          -Compound whole stacks of fruits with whole stacks of organick muck.
          -Recompound fruits/juices until you'll get WHOLE STACK OF COCONUTS.
With some luck after whole night of doing muck and few stacks of fruits you can get 300+ coconuts in a heartbeat. Then just make stacks of coconut oil and recycle whole stacks into crude oil. Easy as a pie.