From a 10% yield i get soon like about 10K of Crude Oil (before the patch is exausted) wich seems a lot to me considering it's an "exausting" patch ... how do i reduce this value?

EDIT: I also made an experiment, created an oil patch with 10% yield left (approx 1600 oil) ... the exhauster still extracted (before patch depleted) around 7600 oil (7.6k). Is this normal?

You're right. 10% is approx 1500-1600, but in fact it's infinite. So I decided to multiply the rest of oil by 5 while exhausting. Now this multiplier is configurable (5 by default).
Updated: [2017.04.18] 0.14.4:
• now oil multiplier is configurable (config.lua). Thanks to xeln4g4 for his comment.