What's the advantage?

 

I'm not sure if there are any advantages. I only went with Idemitsu because it was close to whatever Mazda uses. I may go with something else moving forward. My oil change is due pretty soon.

 

Get Ready for GF-6 Motor Oil | MOTOR

www.motor.com

 

GF-6 might be useful , in engines to use it. A lot of oil industry upgraded performance standards are for reasons other than better lubrication performance, environmental considerations come to mind.
But if your engine's not designed for GF-6 why use it? I've had experiences of GL-5 gear oil totally destroying the synchro operation of manual gearboxes designed for GL-4 due to the former's surface lubricants being too slippery for the synchro cones .
Why take the risk unless Mazda recommend the new oil?

 

From Mobil 1 website:
The primary purpose of GF-6 is to combat problems with Low-Speed Pre-Ignition (LSPI), a common issue with turbocharged engines. As in the 2018 upgrade of API SN to API SN Plus, the new ILSAC GF-6 standards will also require added protection against the harmful effects of Low-Speed Pre-Ignition (LSPI) in today’s modern turbocharged engines.
Low-Speed Pre-Ignition (LSPI) creates excessive pressure within the engine’s cylinders, which can cause permanent damage to cylinder walls, pistons, piston rings, connecting rods and spark plugs. The internal engine damage caused by LSPI can also lead to catastrophic engine failure. Click here to learn more about LSPI and its effects on your engine

Plus some oil formulations out there already meet GF-6 or exceed ILSAC GF-6 specifications but manufacturers have not advertised it yet.