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It'll open up sooner, but your cooling fans play a bigger role in operating temp.
You can modify fan parameters with PCM tuning software and handheld devices. Former usually done by prof tuners, latter done with things like Diablo, Superchips or Hypertech.
I'm no mechanic, but from what I've read in the past it isn't a good idea. The engine isn't able to heat up to proper operating temperature and may run rich and that may not allow you to pass stringent EPA testing. Another thing I've heard is that the oil is more likely to sludge because it isn't getting hot enough to drive the moisture out. Though I'm not sure this pertains to full synthetic oils.