FYI - the difference between Matic J and Matic S is that Matic J is a synthetic blend and Matic S is full synthetic. Both are Dex/Merc ATF.
The belief that Matic J "causes SMOD" is based on Internet Myth citing the fact that Nissan changed the recommended ATF at or near the same time that it began equipping Xterras and Frontiers with an updated radiator. The fallacy of this belief is evidenced by the fact that Nissan changed its recommendation to Matic S for nearly every automatic transmission in its product line, not just Xterras and Frontiers equipped with the faulty Nissan/Calsonic radiator.
Moreover, it is not true that Nissan "won't" sell Matic J anymore. For quite some time after Matic S was introduced and recommended for the Xterra in the U.S., Nissan dealers in Canada continued to sell Matic J. In fact, the Owners Manuals for 2009-2015 Xterras sold in Canada all read: "If Genuine NISSAN Matic S ATF is not available, Genuine NISSAN Matic J ATF may also be used."
Now back to the issue raised in the first post of this thread - In an Xterra with MT there is no reason not to use the radiator for the AT instead of one specifically designed for the MT - just cap the ATF cooling tubes (or not, it really doesn't matter). In an Xterra with AT, any quality aftermarket radiator for the Xterra will work just fine without the risk of SMOD, which in Xterras/Frontiers is the result of a faulty Nissan/Calsonic radiator, not Matic J ATF.