Hi guys, hope the following puts some minds at rest. I am on my 5th Merc. Now and all have indicated oil pressure as you have described, maximum whilst on the move and coming down to 1.5 to 2.5 when at idle in drive, this is normal for these carís so no worries. After you have had the car for a good while you will be able to judge when you need an oil change by watching the oil gauge, I have always found that ( given fairly constant running temperatures ) that as soon as you notice the oil pressure dropping consistently lower than is usually the norm for your car at idle in drive, itís time to change the oil, try it, I found that this more or less coincided with normal oil change periods taking into account the oil used and driving patterns. Yes the oil pressure will rise a little if you change to neutral due to the slight increase in RPM and with no load on the engine from the transmission it will also use a little less gas.
PS I am still having all sorts of fun trying to cure a few ticking noises from the valve gear, anyone out there currently having the same fun let me know.