Use Mobil 1 synthetic, and use the viscosity to suit your climate. In Alberta, most of use 10W-30 in the winter (can use 5-30 or 0-30, but with Mobil 1's super low pour point, 10-30 works just fine) I buy the Mobil 1 at Wal-Mart, as I can often find it on special. Sometimes they have 5 litre jugs on for about $24.00. Even their regular prices are pretty good.
15-40 works well in the summer, but the 10-30 seems to work just as well in these parts, too.
I have the MB oil filter wrench, and it works great. Be careful when taiking out the filter, as there is a wiring harness right above it you don't want to damage.
I use the LiquiVac system, and really like it. It's up to you, but either dipstick extraction or oil pan plug draining are accepted methods.
You can get a proper 1998MY manual from the dealer. Did he buy it from a dealer? for servicing, I have not had good experiences with Hyatt. My Dad lives in Crowsnest Pass, and bought his C230 at Hyatt. Won't take there for service, though. I also bought my car at Hyatt, as none of the Edmonton dealers would sell me a car. Since dad comes here lots, he's been going to Weber with good results, but sometimes he uses LoneStar in Calgary, and they seem okay.
1998 C230 "Black Betty"