In these past few months, I took the BITOG advice and ran DELO 400 in my 535i. My gas mileage went down a bit, but after my engine got used to it, things went to normal. Now my engine is quieter and happy. Therefore, I am happy. My question is, how long should I keep it in? I currently have about 3k on this oil.. I was thinking 7500? Thoughts, please.