I have the m103 engine too and I have a lot of miles on it. Other than switching out the oil pump, I would reccomend changing to 20w50 oil as well as adding a quart of synthetic just to protect the engine at hot temperatures. I just changed the oil on my 300e with the topsider and added the Mobil 1 205 oil filter to some 20w50 mixed w/ Mobil 1 10w30 and my oil pressure stays at 2.5 at idle warmed up and then after intense driving of half an hour it'll drop down to 1.5. If you see it is dropping too low change the oil, I like Chevron 20w50 because I buy it from Costco for 11.xx for a 12pack. Don't be cheap, use a Mobil 1 Filter and you will be happy.

99 s420
97 e320
96 sl600
89 300e