My oil production stated 71k/h. But I waited for 2 hours and I found out it actually increase only..

Oil production (and food production) will vary when you get to the point that you have a lot of troops and they can consume more than you produce.

When you have a lot of fuel the troops will consume it (faster than production) and the amount will go down. When you reach a threshold amount (that depends on your level) the consumption is stopped and will show as 0/h. Your production will continue and the amount you have stored in the warehouse will increase. But only up to the point that you start to go over the threshold again. If you were just below the threshold amount then it would only increase a little before stopping again.

The threshold for food and fuel exists so that you can have a lot of troops and not end up completely running out of resources. You will always be able to produce a small amount to use for marches etc. If you need more of these resources then you have to earn and use the item boxes to top up when required.