If you can't start the engine on MDO, that is because the clearance in the fuel pumps is so exceesive that all the MDO leaks out between the barrel and plunger rather than going to the injector.
If there is that much clearance the problem is unlikely to be caused by temperature differences between the barrel and plunger closing the clearance.
If the fuel is leaking out around the fuel racks where they enter the fuel pump body, when it cools on the outside of the pump it can cause the stickiness you describe, but the washing you describe should prevent that.
There is another problem, completely unrelated to the fuel, which can cause this stickiness.
I am not familiar with this particular engine, but if when the fuel pumps are fitted, the fuel racks are not exactly "square" or perpendicular to the operating shaft, they can mechanically sieze, because the rotation of the operating shaft, will push the racks forward or aft relative to the pumps. The rear face of the pump has to be exactly parralell with the operating shaft and the attachment of the fuel rack to the operating shaft positioned so that the rack is exactly at 90 degrees to the shaft and the face of the fuel pump.
Please let us know how you get on.
It is always better to ask a stupid question than to do a stupid thing.