I think the M54 series engine calls for synthetic engine oil. Agree check your owner's manual. Oil is cheap insurance for long engine life. Check here http://repairpal.com/automobile-owners-manuals-online or try this link you may be able to down load a free copy of the owner's manual for your vehicle http://www.autousermanual.com/
Every 3000 to 5000 miles is a safe bet with good quality oil and quality filter. If use synthetic oil can stretch out interval but this may be cost prohibitive. If the oil is 3 times as expensive as petroleum based oil and you do not feel comfortable stretching out interval by 3 times then does not pencil out. Depending on driving conditions the syn oils will handle extreme cold and hot better but if not in these extremes then petro based oils as long as good quality are no problem. What does your owners manual say?