My 2008 Lucerne, 3.8 v6 (of course) has had transmission serviced (full flush every 60 or 70,000 miles. Now at 250,000 miles it is due for its 4th service . My favorite mechanics are telling me at least 4 different things to do...such as ...nothing, don't touch it...full flush...reverse flush...change with new filter. Color is just beginning to darken but still reddish-pink, shifts perfectly...getting 30+ mpg. Help!