I just did the flush based on a couple of conditions. See that they have an experienced tech do it and not some rookie. At my dealer they use a different machine for each type so there were no cross contamination issues. The tech that did mine has not seen a problem flushing in 16 years. For the extra 100 I did a full flush which I will most likely do every 20000 from now on but I don't drive many miles and I drive it like it was rented, lol.