This happened to me on my 2008 X5 4.8i. I was 30 miles into a 175 mile trip. Of course, I did the mandatory pull over immediately and stop engine to investigate. I quickly determined that I had not lost oil, the engine was not making collapsed lift noises and thus concluded that the oil pressure sending switch was at fault. I continued on my way while doing rpm experiments. Above 2500rpms, the sender would work fine. I replaced the sender yesterday and all is well again. My 2004 545i needs its third sender due to leaking oil and making a mess.