over here in Canada both redline and amsoil are comparable in price around the $20-22 per qt before shipping. thats not a bad price out my way. I haven't heard anything bad about either brand but am doing more reading on redline as i haven't heard a lot about it. so far it claims to lower operating temp, some forums say it gets them better mpg and its apparantly won't work in some transmissions because its so slick.
people that tow a lot like redline and amsoil interchangeably.
I source Valvoline gear oil from either Advance, Menards, or Walmart depending on sales, rebates and discount codes for the best price, and have used Supertech to cover a partial quart with no adverse effects.
Amsoil would probably be your best bet for a synthetic 80w-90, which is particularly hard to find on a shelf, otherwise you are limited to a conventional 80w-90 or have to go to either 75w-90 or 75w-140 for something synthetic.