Man did you buy the wrong car for that kind of driving. You get almost none of the benefits of owning a diesel much less from a tune. You won't see less wear from changing the oil earlier. You have to find out why the extra metal is being produced. You are using a proper oil so that isn't the issue. It is made for 10k changes+. I would venture to say it is your short trips that may be causing the excess wear metals. That is really hard on any engine. Being under 100k isn't it either. When it was new that may have been a factor for the first change or two, but not now.