Originally Posted by Diesel911

I think I have found an answer; In the MB service manual (18.8-005/2) it states the relief valve in the oil filter housing is set at 3.5 bars (on my 84 300D). Since the oil returning from the oil cooler goes back to the oil filter housing and through the oil filter part goes to the engine bearings and part goes back to the oil pan.
This indicates to me that as long as everything is functioning correctly (especially the oil filter relief valve) the oil pressure in the oil cooler and oil cooler hoses should not be more than 3.5 bars.