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I have a 2007 Toyota Corolla, and the owner's guide recommends GL-4 or GL-5 75W-90 for replacement oil.

I understand that you get smoother shifts and lower fuel economy with higher viscosity oil. Are there trade-offs with fuel economy between the different API ratings? Should I consider some of the more expensive synthetic options from Royal Purple or Amsoil?

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Generally, fuel economy improves slightly with API improvements. The main improvement in GL-5 is its extreme pressure protection, which in theory might help fuel economy.

You should most definitely consider synthetic oil. Mobil1 is usually rated better in most categories than the brands you mention. so I've used Mobil1 Synthetic Gear Lubricant LS 75W-90 in my Mercedes-Benz.

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