C320 OIL CHANGING
First of all, "Perform service on time" will display when the engine hasn't been shut down long enough for an accurate level check, in the opinion of the oil level sensor. I also see it when the oil temp isn't high enough.
I have changed the oil once in my '01 C320, using my Topsider down through the tube with a cap/cover at the right rear. It looks like a transmission dip stick tube. It isn't since there isn't one or at least I've never found it. The vacuum procedure gets all the oil out, verified by measurement. The next oil change (to Mobil 1 0W40) will be at the long awaited B service which looks like somewhare around 13500 miles. It was last changed at 7200 miles. Although Mobil 1 0W40 was original fill, I'm using Mobil 1 15/50 since 0W40 could not be bought at retail except at Porsche and MB dealers at a hellish price.
When the car was new I purchased the shop tool dipstick (one size fits all models, apparently) at a steep $110 which I use once in a while just to verify that the built-in system hasn't gone south on me.
Cars like this are frustrating for the car hobbyist since there is nothing mechanical to do. That's why changing the oil early is so fulfilling. I think I miss, just a little, all the warranty visits to the dealer with my past MBs. After all, I owned two 201s and we all know what that means.