Transmission fluid ATF+3 vs. Dexron III / Mercron

Are the two compatible? Whats the real difference? I recently changed my fluid on a Dodge Ram 2500 (2003)with a 48RE transmission built by Suncoast transmission. The fluid that my mechanic put in it was ATF +3 when it was installed. I called Suncoast and they recommended using Dexron III/Mercron. I did not flush the system as I should have and believe that there were several quarts of the old fluid left in system. Is this going to be a problem? It shifts ok and there are no signs of any problems. This is a very expensive tranny and I need to be sure.

No, these are NOT the same, and your transmission requires Chrysler-specified fluid. ATF+3 is no longer being made, and has been superseded by ATF+4 in all applications. It is synthetic-based, and has more resistance to heat and breakdown than ATF+3.

You must have talked to a general flunky at Suncoast transmissions. Anyone who knows anything about fairly late Chryco trannys knows that they require ATF +3 which has now been superceded by ATF +4. If indeed THEY told you to use D/M, then they owe you a complete flush and fill. Do it ASAP.