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.