Yippee! Another oil thread!
Kobeck, 1800 miles was a little premature, but that's OK. It will probably turn black quickly but this is normal - don't worry about it. The safe interval is determined by oil analysis and will vary with each different engine, although generally OM61x motors produce more soot and can't go as long as OM60x. You didn't mention what year/model you have (that's nice to put in your signature, along with your name, btw.) Anyway, 5kmi is probably a conservative estimate for future changes, although I wouldn't go beyond 7.5k or 10k without oil analysis to back that up. My analyses show that my 603 engines can go 10-15k safely while my 617 can go 7.5-10k at most, assuming a 2% soot limit (that's Mercedes limit, the Delvac-1 can suspend 4%, while Mobil-1 can only suspend about 2%.) For oil analysis, I buy kits from Snider:
http://www.sniderpetroleum.com/oilanalysis.html
Specify the "extended drain" kits, $12.50 each plus S&H - it's cheaper if you buy several at a time.
Best regards,