I did trade in a 2015 GT for my 2016.
If you don't mind me asking, what made you go from a 15 to a 16? Not trying to derail this topic but I literally just bought a 15 3 weeks ago.
For on topic I just let the dealership put synthetic in it, I've never heard of additives added but usually go on the dealerships recommendation which is usually 3-5k miles.
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