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.
Having a signature is like keeping a gremlin.
Just don't overfeed it, don't make it wishy-washy and you'll be a good responsible Signature Keeper :]