Gear oil viscosity is defined in SAE J306, which has been revised.

SAE J306 Specifications - The Lubrizol Corporation

SAE 90 gear oil is about the same viscosity as SAE 40 motor oil at 100C (212F). Adding the 80W to it just means that it has been tested to barely flow at -26C.

Note that 90W does not exist, even if Star Brite distributing incorrectly labels its product as such. Straight 80W, which I have never seen, is about as thin as SAE 20 motor oil at 100C.

As far as the word "hypoid", that term has been replaced by API specification GL-5.

So, you can buy a special marine product, which may or may not have better corrosion protection, or 80W-90 GL-5 automotive gear oil, which costs only about $4 a quart.

Jim