I have oil patch near base with 60 yield, already pumped few millions from it, and its not decreased a bit? why so and how to make it like in vanilla?

RSO doesn't modify minimum yield in any way I think.
There is a chance that since that base game mechanic that increases minimum yield based on initial one worked on it. I'm not sure how it works exactly so I can't confirm that in any way.
Is rso only mod you have or there might be others that modify the oil wells?

Only RSO, nothing else modify oil

Please post exchange string I'll take a look.