The oil change interval recommended by BMW (6,000 miles) is probably a little bit conservative as it is, I would recommend that be followed.
There are many very good oils on the market that meet BMW's specs (SAE 5W-40, API SL / JASO MA2).
Motorex is fine, I think Bel-Ray is the best and Shell Rotella T6 is probably the most popular.
This is a list of suitable oils that I compiled that meet BMW's specifications for the new liquid cooled machines:
Oils meeting BMW Specifications (API SL, JASO MA2 and 5W-40):
Motul 300V Ester Core
Motul 7100 4T (Ester)
Bel-Ray EXS (Ester)
Bel-Ray Scooter Synthetic Ester Blend 4T
Shell Rotella T6
Motorex Boxer 4T
Motorex PowerSynt 4T (PAO/Ester)
Motorex Top Speed 4T
Spectro Platinum 4
Liqui Moly Racing 4T Synth
BMW Advantec
Castrol Power 1 4T Racing
Silkolene Pro 4 Plus
Silkolene Quad ATV
Ravenol Motobike 4-TAKT Fullsynth (Ester)
Repsol Moto Racing 4T (orange container)
Kawasaki Performance ATV/UTV (semi-syn)
Petronas Syntium Moto 4SP (not sold in U.S)
Delek Motor Cycle 4T (not sold in U.S)
Eurol Sportbike (not in in U.S)
Elf Scooter4 Maxi City (not sold in U.S)
Olio Motore 4T Emi (not sold in U.S)
Gulf Syntrac 4T (not sold in U.S)
Zuminol Double Ester (not sold in U.S)
Ulei Moto 4T Ipone Full Power Katana (not sold in U.S)
Eurol Sportbike (not sold in U.S)
ER 540 Infinity Energy (not sold in U.S)
Kroon-Oil Expulsa RR (not sold in U.S)
Freedom 4T Motorcycle Oil SM (not sold in U.S)
Rock Oil Synthesis 4 (not sold in U.S)
Bardahl Racer Fully Syn 4T-N (not sold in U.S)
Champion Pro Scooter 4T (not sold in U.S)
There are other viscosity oils that are JASO MA2 and API SL (or higher) out there that are suitable but I wouldn't recommend straying too far away from the 5W40 weight that BMW recommends.