Hi Scotty.
Can I use diesel oil in gasoline engine?
Car mechanic asked me which "C" ratio (c1,c2,c3..) oil to put, the manual didn't say anything about it (only 5w-30) so he put C3 5w-30. When I came home and made some research, I found out that "C" ratio is for diesel engines...
So should I change the oil again? Or can I use that oil in gasoline engine?

NO, each has it's own additives. Now in a pinch you could, but not a good practise. I would change it now

Thanks a lot, I'm not from America but still watching your videos.