Hello to everyone, since this is my first time writing in this forum. recently i bought a 1995 E320 and a 1987 190D from a guy who worked himself on the cars. I plan on doing the same thing, and this weekend I want to change oil and filters on the E320. From looking in his receipts, he used the same oil type on both the 190D (which is a diesel) and on the E320. The oil he used is Mobil Delvac 15w40, which is for diesel cars as far as I know. The E320 engine has been running on this oil for a few years as the receipts tell me. I live in an area where it snows 3 months a year (South Dakota). Should I continue to use this oil, or change to a 10w30? I don't really want to change the oil brand since it always has used the same oil, but if I have to than I will. Also sometimes when I turn on the E320, i hear this little knocking noise (like the noise a diesel makes, but not as loud), not always, only sometimes, especially in the mornings. Could it be from the oil or oil filter (it has Mann oil filter in it, and that's what I plan on using in the future), or is this normal for its age/mileage? Other than that the car is in top shape. Any comments would be greatly appreciated.