You want a gear oil that is rated GL-4. The later ones, such as GL-5 have detergents in them that attack the yellow metals in a Toploader, such as the blocker rings. One that I've been using in my Toploader for over 30 years now is:
Amazon.com: CRC Sta-Lube API/GL-4 Multi-Purpose Gear Oil 85W90, Sta-Lube, 1 Gallon, Hypoid Gear Oil, Yellow Metal Safe, Anti-Rust and Anti-Wear Ep Additives : Automotive
Amazon.com: CRC Sta-Lube API/GL-4 Multi-Purpose Gear Oil 85W90, Sta-Lube, 1 Gallon, Hypoid Gear Oil, Yellow Metal Safe, Anti-Rust and Anti-Wear Ep Additives : Automotive
