The easiest places to find them from buildcraft are in deserts and oceans I don't know if this apply's to galacticraft, but builcraft oil is compatible with galacticarft items.
Interesting. I've certainly gotten an expanded perspective on the matter.
Of what I know is this issue began when the oil was rewritten in build #315. If you'd load an old world that already got oil generated with a newer build of Galacticraft, it would expand into a black mess that would no longer be capable of interacting with any mobs nor players alike. However, I did manage to use an oil extractor on a presumed source block. It allowed me to perform an infinite extraction of oil. Although, if I stopped the process, it appeared I couldn't extract from that source anymore. I'm also suspecting some original oil deposits that was untouched could have vanished as well, but I'm not sure.
Anyway, here's a visual example:
This is how exposed oil deposits could typically look like in build #314. It works like a charm.
When I loaded the same world again with a later build (this one being #410 [which is also the last 1.5.2 compatible version]) the oil will expand and it will lose the ability to physically interact with mobs as well as not being able to drown you anymore.