I've owned my GR for a year now. It gets super hot here in the summer plus Autox. I'm thinking of switching out of the OEM weight. The Manual does say 5w-20 can be used.
Also, based on this video. This Engine loves to be hot. I noticed this last summer while driving under heavy loads. I felt the car get quicker. It happened again last weekend. Or I'm going completely insane and my brain thinks it's happening.