Yes you can use TranSynd in your 4L60E. TranSynd is basically a synthetic DEXRON III(although DEX III is no longer licensed by GM). The early 4L60e transmissions were spec'd for DEXRON III and later went to DEXRON VI when that became GM's latest and only licensed ATF.
TranSynd is a very heavy duty ATF(why its used in allsions), but GM would never endorse its use in something like your transmission for obvious monetary loss.
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