If one is using 5w-20 GTX full senthetic oil how long should one go between oil changes. 2007 m3 with 2.3L eng. Right now I am planning on changing every 7500 miles with a oil filter change in between at lets say 3700 miles. It will take me appx.7- 9 months to accumlate the 7500 miles. Any advise about what I'm doing.
O, I have installed the oil filter adapter that's used used on the 2.0L eng. And now use Fram spin -on filter.

 

Everything sounds good except the mid-point filter change, which is a waste of money and will not add even a single mile to the life of your engine. FWIW, as oil filters age, the filter better and better, right up to the point where they clog (which shouldn't happen for at fifteen thousand miles *or significantly more* on a properly operating engine).

For the duration of your warranty, simply follow Mazda's recommendation of 7,500 miles for both the oil and filter, and as the warranty is running out, have the oil analyzed to help you determine how much beyond the recommended OCI you can run your oil and filter combination. My bet is that with the oil you're using, ten to twelve thousand miles will ultimately prove to be an appropriate OCI.