You should be OK. BMW has an oil condition sensor in the sump (a Continental Temic). The principle of how these gizmos work is explained here -
http://www.machinerylubrication.com/...s-oil-analysis, and here -
http://www.machinerylubrication.com/...filter-sensors.
You can't tell much by looking at the oil filler cap. Condensation often collects there when the engine cools, particulalry in cool or cold weather, and that can give the oil film on the inside of the cap the milky or muddy appearance you noted.
By the way, I assume you've got a '13 or earlier model year that is on the 15k mi oil service interval.
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'14 328d, '13 M3 ZCP, '12 535i, '11 335d, '09 Z4 DINAN Stage II, '05 330i ZHP, '05 M3 ZCP, '02 M3, '91 328i, '90 535i, '88 528e, '88 M5, '87 M6, '95 Z3, 95 M3, '93 M5, '87 325e, '89 535i, '86 635i, '73 Bavaria.