No , you should use what is recommended in the owners manual
You can use any SAE certified engine oil in your Nissan. The engine oil should be a 10W-30 weight engine oil.
Use SAE 30 weight. I highly recommend you use Synthetic oil in any air cooled engine.
Any Premium 10w30 weight oil.
5W30 weight only. If you run any heavier oil in that engine it will burn the valve train up.
Use any 15-40 W with JASO MA or MA2 certification. Equally, an oil of same weight with SAE SF or SG certification. It may be possible to use a 10-30 W oil if the engine is in prime condition, or a single weight 40 oil if the engine is very worn.
40 weight
5w20 weight.
Any premium brand. The weight I cannot tell you as you did not list the year. My 1995 LTC calls for 5w30 weight oil...
You must ues 5W30 oil. Any heaver weight oil will not lube the valve train correctly and damage the engine.
No, absolutely not, that is the wrong weight oil for your Civic. Use 5w30 weight oil. You can use a full synthetic but use the correct weight if you want this engine to last.
No, use only the weight oil as recommended by the manufacture.
the weight of oil viscosity weight is strictly related to temperature of operation. there is no set rules for it since anew engine will benefit from less dense oil an old wore out engine will not.