From the picture, look like you strip the GPU. You need to put back the radiator. You strip the GPU only if you immerse it in 3M Novek fluid.
For mineral oil and other fluid except 3M Novek you put the hole GPU assembly (Without fans or whit them depends of your choice and cooling) and make a circulating cooling system for the mineral oil.
3M Novek is a different story altogether you can research it, but the setup cost are very high.
Hope it helps
Looks like you 're right, thank you!
I did not expect the thermal conductivity of copper(radiator) and miniral oil to differ sooo much.
I have a circulating pump in tank, speed about 1.3 - 2.5 cubic meter an hour, but this is not enough for good mixing inside the tank.
Do you think it is real to run a strip GPU in mineral oil if to make good mixing of mineral oil in the tank?