Motor oil has had insane amounts of development and funding put into it, likely billions, whereas basically all gun oil is rebranded or a rebranded mixture of existing things. Keeping billions of vehicles running is more important than most other things ever. Motor oil is high temp, can be as thin/thick as desired even below freezing, cheap, and is specifically designed to reduce buildup and wear on metals.
If it's not $20, make that ol fudd's day if you want to.
Your oil probably has molybdenum or graphite dust in it, which is good and itself is a great lubricant. Guns are dirty, so a little extra oil isn't gonna make them any worse.