Manufactures want thin oil for better fuel economy. That's all.
I went 5w30 when I added forced induction because all Subaru engines that are forced induction come with 5w30.
If I was tracking the car then I would consider an oil cooler and a heavier grade oil. The GT86Cup manual has the oil listed as ELF HTX 825
10W60. There is an oil cooler option. This is for "stock" cars, so no forced induction, which means less heat and less forces pushing down on those bearings.
https://racing.tgr-europe.com/wp-con...anual_EN-1.pdf