The difference in viscosity between 50wt, 40wt and 30wt is neglible at 212F. For example 50wt is 23cst, 40wt 16 cst and 30wt 11 cst. On the cold side however there is huge difference, 600 cst, 400 cst, 300 cst at 32F. Use any and it will work. Advantage of the 15w50 is it has high
zinc and phosphorous, but you can get the redline break in additive and get what every you want. If I was going to have hard track days the 15w50 would be the choice, anything else 30 or 40 with the additive.