A couple of weeks ago, on a particularly cold morning I got the 'Low oil pressure.... stop carefully etc' warning. My heart sank having spent a small fortune over the last couple of years on several major repairs. However, I turned the engine off and back on and the warning had gone. This has happened a couple more times and I went to a local BMW specialist. They couldn't find anything wrong but did say any repair if needed would entail lowering the subframe or engine out - 6 hours work. It's happened a few times since but only when the outside temperature is low and never when the engine is warm. The car has a full and complete service history, had £1000's spent on it, done 156k miles and is a 520D. Should I be worried?