I've used 20/50 Mobil One and Amsoil (both synthetic) it's a slightly thicker viscosity and stays put a lil better than 5/50. The only drawback to Motor Oil is the smell when it gets hot.
I feel there is better out there.... but it will work if your just wanting to cheap it.
Like Brownie65 said, there is a ton on this forum for it.. do a search and you will quickly see that Lube is a personal preference and everyone has their favorites and they are typically all different. This can be based on location, from mild climate, to salty climate to freezing climates... just use the best for you and your condition.
Hope this helps!
The Lubes I've used on my firearms are:
SuperLube with ptfe (Grease)
Froglube (paste better than oil)
20/50 Amsoil Synthetic
20/50 Mobil One Synthetic
Lucas Red/Tacky Grease
All are good...
lately I've been mixing some SuperLube Grease with a little Weapon Shield Oil and I am loving it... super slick and stays put.
to each his own... Lubes are like asking if people prefer Brunettes, Blonds or Red Heads everyone has a favorite, none better than another. It's all pink on the inside.