It's better than nothing but yes it's too thin. Most likely it will be burnt and dispelled out of the exhaust. It may also be pumped out of the breather system.
In an emergency, no. Change the oil ASAP. If it's an emergency.... only if you have to do so. It can hurt it. Diesel oil (15w40) is different from 10w30. There are different additives, different qualities, and the viscosity is also different.
10w30 10w30
It depends on the situation. Personally, I wouldn't use 5w20 instead of 10w30 for a complete oil change. However, using it to top off a quart or two shouldn't be an issue. Oil shears over its life so a little thinner oil shouldn't hurt the engine.
Every car needs a certain type of oil. The manufacturers usually tells you what kind of oil to use. Putting 10w30 oil in a 2005 corvette could really hurt the engine but putting it in a 1999 to 2004 Chrysler 300 M is perfectly acceptable. Considering you didn't say what model of car, this question is very hard to answer. The simple answer is to use the oil grade recommended by the manufacturers of your car.
10w30 is thicker than 5w20
10w30
SAE 10W30 or yamalube 10w30
It shouldn't hurt anything until you get into the colder months. But, 5w30 is the recommended oil for that car. 10w30 is also a suitable oil down to 0*F.
5w30 or 10w30
10w30 if using regular oil. I recommnd Spectro 10w30 clutch saver oil.
10w30
5w30 or 10w305w30 or 10w30