Debating between Castrol Syntec and Royal Purple Max-Gear in 75W90 for the rear differential and the transfer case for a 2019 Hyundai Kona AWD. Currently have 37500km and will be doing a fluid change at the next oil change. Anyone have any experience with either Hyundai AWD on-demand system for what they used? Or which of the two I have listed would be better for a 40,000km interval and why? No, I am not interested in Hyundai OEM gear fluid. Thanks.