Hard tellin' what kinda crap was in the crankcase originally, and I've been wholly under-impressed with Chinese filters. Your initial pressure could have been as a result of sludge and/or a blocked bypass valve.
Everybody has their personal preference in oil and filters. The main thing is to use a filter that is specified for use on diesel engines. I wouldn't use one that has all gasoline engine applications. And make sure the Service Rating on the oil starts with C.
Then adjust the oil pressure sending unit. It's a pressure rheostat (a round silver thing) sticking out of the engine block, lower left side. One wire comes off it, heading up towards the dash. Rotating it left will lower the indicated oil pressure, right will indicate higher. With the (cold) engine OFF and the ignition switch to RUN, rotate the OPSU so the needle is dead center on the zero pressure mark. That should calibrate it for future reference.
Just so there's no confusion though, this has NO affect on the actual oil pressure itself. The procedure is simply to adjust the OPSU for the most accurate indication on the gauge.
//greg//