Most people don't realize it, but 80-90w gear oil, and 30w engine oil, are the basically the same viscosity, at room temp. So is hydraulic transmission oil. This fact upsets some people, but if you do your research, you will see it's true.
The viscosity varies slightly, depending on the brand. But, they come in around 30w, on the motor oil scale.
The difference between them, is mostly the additives. Same with a cheap oil vs, a good oil.
The gear oil scale is NOT the same as as motor oil, giving people the impression it's heavier, because it's 80w. It is not. Pour them side by side, you can easily see this.
The 80-90w lawnmower engine story doesn't sound right. It's hard to get an engine to throw a rod in ten minutes, even if you drain all the oil out. Plenty of YouTube video's to illustrate this. If it indeed happened that way, it was ready to throw the rod anyway.
Gear oil is of course entirely wrong for that application, but, it's still lubrication.