Motor oil may work, but hypoid cut gears found in the 8.25" differential require difference properties than helical or straight cut gears found in manual transmissions and transaxles. If it's relatively light duty application then a good motor oil might last.

If it's an experiment then go I wouldn't bother with slick 50. I would go straight to a 0w20.

The 80w90 gear oil has approximately the same viscosity as 30w or 40 w sae motor oil.

Take a looky.
https://www.kewengineering.co.uk/Auto_oils/oil_viscosity_explained.htm