Intensity 6000 for an oil patch only add +2% yield amount, for both spray and brush. (Size 1)
I tested and if you change the size the amount added also increase for the same Intensity. This may make sense for spray but not for brush.

I used to have Endless-resources mod and when I updated to 0.13 I turned off the mod to get some achievements, but the game still count as "Modded". So I removed the mod in the factorio folder, but the mod still appear in the game, and all my games are "Modded" so I can't get the achievements. Tryed ...