I think this may be a bug. I have one oil field that is filling the appropriate trucks with no issues. I have another that will just not fill trucks. I have no idea why. I have compared the two and can see no difference.
Keith
I assume you're using pak64, version 102.2.2 so set me straight if my guess was wrong.
1. Is the another Oilfield connected to it's appropriate Consumer? In other words, have you made sure you're trying to transport materials between a working factory pair. (http://en.wiki.simutrans.com/index.php/Transportation_basics#Finding_a_factory_pair)
2. Is the cargo station already full of oil with destination elsewhere?