How old is that recommendation? The 75w-90 scale was split a few years ago (2003 IIRC) and the thicker half became 75w-110. There aren't many 75w-110 gear oils on the market but I know Amsoil makes one, and their gear oils are darn good stuff.
If you want to go with 75w-90, I would highly recommend Motul Gear 300. That's used straight out of the bottle by full-on race cars. Can't get much better than that.
If you really want to go thicker, you'll want the best stuff you can get so that your temps stay low (thick oil doesn't cool as efficiently). Castrol SAF-XJ would be my recommendation. You can get it from BMW or Nissan dealerships since it is factory and service fill for all the BMW M cars and the Nissan GT-R.
Or, you could just stick with the GM fluid. It's probably fine, to be honest. But that's up to you.