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GM's testing on this engine does not require the use of synthetic oil. It won't hurt, but not necessary. As far as temp decreases, it is minimal, and remember, for efficiency, the engine must be running at a certain temp. The true advantage of syn oil is in extreme hot conditions (resistance to breakdown) and extreme cold conditions (flows at extreme cold temps).
If you want to find out how well non-syn oil is doing, send a sample for an oil analysis and see how the TBN is, to check the true oil life remaining.
For some people, using syn oil is similar to burning premium fuel.