Ran a R42 for eight years. Never noticed pressure drop once. Always ran high on gauge. Our experience with our Deutz motors was that they always seemed to use a little oil, not really a bad thing we feel. Using same oil you are,and have for years.
We have found if you run the oil at the full mark it will lower it in only a hours running. I have 3000 hours on a r50 deutz and after filling to only a 1_8" over add mark cut my consumption to a quart every 10 hours. Be careful of this as yours might not stop at the add mark. Don't want to cause any problem but try this and check oil every 2 hours first day. Remember to let stand for 5 mins and push the dipstick in and wait 5 seconds before removing. The o ring seal will push oil out of the bottom if you do it fast. Our oil consumption does increase as it gets closer to oil change time. A dirty air filter will also increase oil consumption on a 913c deutz.