I read everything ppl wrote and used it as reference where to build my pile. So I made it in middle of empty area, closest tree is 12 tiles away diagonally. it took 5days 20hours to dry (didn't know it can take that much over 5days, so those 20hours were longest 20hours in game XD). I got 11/40 dry boards. from those 11 dry I got only 2 planed ones... I guess with this luck I would get 0 oiled ones

I cleaned one pile from neighbour who stopped playing. he placed it next to a tree, 4tiles away is river. in like 10x10 area with pile in center are 2 drying frames, house, and lots of containers. it's grassy area that is not flattened. and guess what? all 40 boards were dry

so maybe passing many times near pile really make moldy ones (since he doesn't play no one passed there)...
I'll make new pile on same spot to see what will happen XD