FYI
HKS recommends a 40 weight engine oil:
https://www.hks-power.co.jp/en/produ...arger/db/20246 (under installation notice)
Here, in Europe, specifically in Germany, most HKS GT SC users use 5W_40 or 5W_50 from Motul or Ravenol (excellent engine oil, group IV/V + Tungsten/Molybdenum additive, also used by Gazoo racing).
I use 5W_40 (Ravenol RCS) with my HKS GT2 SC (including HKS oil cooler) / built engine combo.
Cheers