i always use Fuch pro race fully synthetic 15w50 on tweaked Sti 2.5ltr and high tuned 2.0trs. As advised by some tuners. But PPP 2.0ltr WRX 2.5ltr ok on 15w40. I dare say 15w40 would be ok in above. I have used Motul.
6 years thrashing my Hawk T20 around tracks it never complained. i changed it either just before or just after a trackday, depending on milliage. So my average oil change was 1,200 miles.
Now my T25 430bhp has an oil cooler and less temperature variation, my oil changes are more like 1,700-2,000 miles.
My 2011 2.5 370bhp has forged pistons so be OK 15W40 but its run on Fuch 15W50 as easier to keep one oil
Cost of oil, filter labour under£70, so peanuts compared to a rebuild. So most people get given some advice and if works out, they stick to that plan, even if technically incorrect.
Tony
Oilman will give more wisdom than us plebs