When two metalic moving parts meet there is abrasion from friction causing wear and heat. Oil on the surface causes less friction between the parts and thus the reaction between the two parts is deadened.
5w-30 oil in your choice of manufactures.
Most manufactures recommend SAE 10W30 or 30W. I recommend you use 10W30 Synthetic in any air cooled small engine.
No you can not use gas engine oil in diesel engine oil because processing of diesel engine is different from gas engine oil. If you use this then it damage your vehicles.
the engine oil to use is 5-w20
Use any good grade of motor oil that meets the manufactures specifications. Use 5W-30W oil.
What do you think will happen when you ignore the manufactures recommendations and use a weight of oil they do not recommend? You will have premature internal engine wear. It may take time but you are causing wear. Use exactly what the manufacture recommends.
NO, do not use diesel oil and a gasoline engine.
what is a good engine oil to use in my car
You don't. Add not additives to your oil. If additives worked the manufactures would add them to the oil and recommend you also do so.
I change mine every 5,000 miles but you should follow the manufactures recommendation.
You can use any SAE certified engine oil in your Nissan. The engine oil should be a 10W-30 weight engine oil.
Use exactly what the manufacture of your vehicle recommends. All manufactures recommend a multi weight oil such as 5w-20 as an example. These oils are 5 weight at startup and 20 weight when the engine gets warm.