If your sure it changed when the heater was turned on and didn't come back to higher point when heater was shut back off I would go back to engine and help cable return all the way. That is a very long cable and could be a bit sticky. The cable is hooked to engine at left rear corner as you view engine climbing up the steps. If that don't do it you may want to cycle heat control on and off a few times to see if it shapes up. I don't know if that Deutz has some of the valves the small inline six has that can affect oil pressure. I would have to study up on that if you actually have a low pressure problem. May want to plump in good electonic gauge to see if you are within specs. Good luck