Mixing ATF's? Mobil1 type?

Those of you who use Mobil 1, which ATF do you use?
Their website says both "ATF D/M" and "Multi Vehicle ATF" can be used to replace Dexron III. Which is better? What is the difference?
Also, if you have previously used a different brand (i.e. Royal Purple) is it necessary to completely flush out all of the old fluid to avoid any possible problems from mixing incompatible fluids? Or can they be combined?
I ask this because I have heard it is good to drain and fill (at 15K miles) in between doing a complete service (every 30K miles) when the pan is dropped to replace all of the fluid as well as the filter and gasket. Thanks in advance for any information on this.

Even dropping the pan, does not get the couple of quarts of juice outta the torque converter. In most vehicles there is no way to drain it, so as long as you're using the proper type, mixing the small amount left, with the new stuff, won't present any problems.